To answer the question from the OP, if the world is old, what changes about God and His sovereignty? I would say nothing changes. Why? Because as it is written:
Malachi 3:6 For I am the LORD, I change not; therefore ye sons of Jacob are not consumed.
Hebrews 13:8 Jesus Christ the same yesterday, and to day, and for ever.
Scripture makes it clear, God does not change.
I believe the best approach to creation views within Christianity is the same approach that should be used with eschatological views within Christianity. There are differing views on how and/or why God created the universe and there are differing views on how and/or why God brings about end times eschatology. Remember, that Jesus did not tell Christians to go out into the world and preach creation views or eschatological views. He told us:
Mark 16:15 And he said unto them, Go ye into all the world, and preach the gospel to every creature. That Gospel is that Christ died according to the Scriptures, that he was buried and that he rose again according to the Scriptures. (1 Cor 15).
If someone is looking for spiritual truth regarding how and/or why God created things or explanations into why the Universe began or why anything exists in the first place, then I believe a good Christian response would be to claim that there are varying creation views throughout the history of Christianity and to take each view and fully explain it (even if one does not personally agree with certain specific creation models), explain each Creation models strong points and weak points; where it succeeds and where it fails and then let the person asking the question decide for themselves as to what they believe is the best model. As you know these views include scientific based Creation approaches, exegetically based Creation approaches and Creation approaches that use both science and exegesis which include :
Day Age Creationism
Theistic Evolution Creationism
Progressive Creationism
Scientific Creationism
Ruin/Reconstruction/Gap Creationism
Cosmic Temple or Framework Creationism
Young Earth Creationism
Historical Creationism
Apparent Age Creationism
Punctuated 24-hour Creationism
And probably a few other lesser-known creation models I can't think of at the present moment.
I believe taking this approach would reduce a lot of argument within Christendom about beginning and end time views that are not salvation or fellowship issues whatsoever and yet still keep active a healthy debate between new Christians, long time believers and nonbelievers IMHO.
My own personal creation stance is ruin/reconstruction also known as Gap creation, which is an Old Earth model of creation which I believe is the most comprehensive and cohesive of the creation models. Most criticisms of this creation model are based on the understanding of Exodus 20:11, that the LORD made the heavens and the earth in six days.
Exodus 20:11 For in six days the LORD made heaven and earth, the sea, and all that in them is, and rested the seventh day: wherefore the LORD blessed the sabbath day, and hallowed it.
From a ruin/reconstruction/Gap perspective, the six days that are being spoken in the above verse are the first six days of the Second heaven and Earth Age. The ruin/reconstruction model claims that there are three ages. The first age which lasted for billions of years and due to Lucifer's rebellion was destroyed by God. The second age which we are presently living in which begins at Genesis 1:3 and the third age or the age yet to come, spoken of by Peter in 2 Peter 3:13; the new heaven and earth.
Another criticism of the ruin/reconstruction model is that it claims that there was death before the fall which seemingly contradicts Romans 5:12:
Romans 5;12 Wherefore, as by one man sin entered into the world, and death by sin; and so death passed upon all men, for that all have sinned:
However, if the context, especially the subject of Romans 5:12 is to be taken seriously, the subject is that death, specifically human death, resulted from sin. Not necessarily the death of plants and animals.
The Bible's description of Lucifer says some very interesting things. Namely:
1 John 3:8 He that committeth sin is of the devil; for the devil sinneth from the beginning. For this purpose the Son of God was manifested, that he might destroy the works of the devil.
And
Hebrews 2:14 Forasmuch then as the children are partakers of flesh and blood, he also himself likewise took part of the same; that through death he might destroy him that had the power of death, that is, the devil;
Over the years, I have looked at literally every exposition regarding that phase, "him that had the power of death". I came to the conclusion a number of years ago, that what it is signifying is that Lucifer had the power to cause things to die. I believe God gave him that power but he misused it to corrupt God's creation. If we take the fossil record on this planet and look at it as evidence of an Old Earth, we can conclude that for hundreds of millions of years, plants and animals lived. And then they died. Why? Because a 1 John 3:8 stipulates, the devil sinned from the very beginning.
Another criticism of the ruin/reconstruction model, is that it simply cannot be true because the Hebrew word, "Yom" as found in the Genesis account,transliterated as our English word "Day" means exactly what it means. A 24 hour day.
But in truth, there are at least 5 different meanings for the Hebrew word for Day.
1: In Genesis 1:5, God called the light Day. Compare to John 11:9 where Jesus claims, 'Are there not twelve hours in the day?". A 24 hour day is not the view here. What it is signifying is "Daytime".
2: The second time the word day is is used is again at the end of the same verse, Genesis 1:5, where there was evening and morning the first day. This certainly does signify a 24 hour day.
3: The next day we need to pay special attention to is the account of the Seventh day in Genesis 2:2. There is no mention of evening and morning. If we ask how long God rested from his work, then Augustine's suggestion that God sanctified the Seventh day, making it an epoch of an unspecified time that extends towards eternity makes sense.
4: Another meaning of "Day" is found in Genesis 2:4, "in the day that the Lord God made the earth and the heavens,". This is used as a figure of speech, combining all the days of creation into one phrase, "in the day", which does not refer to a 24 hour day but which clearly refers to periods of undefined lengths of time, such as the phrase, "the last day".
5: Finally, there is the word day as used in 2 Peter 3:8:
2 Peter 3:8 But, beloved, be not ignorant of this one thing, that one day is with the Lord as a thousand years, and a thousand years as one day.
This word for day here compares one of the Lord's days with either a thousand human years or that a thousand years is used as a very long period of time. At the same time, it claims that a thousand years are "as" one day with the LORD, simply meaning in my opinion, that time means nothing to God.
I think it would be very closed-minded for any Christian to claim that the meaning for the word Day means the same thing throughout all of Scripture.
I think it would be to everyone's advantage if they picked up professor John Lennox's book, 'Seven Days That Divide the World: The Beginning According to Genesis and Science.' I found it to be a thoroughly educational and entertaining book.
I hope that helps.