those who used to be risen in him.
The passage leaves no place for that concept, someone who "used to be risen".
Colossians 3:1-4 KJV
1) If ye then be risen with Christ, seek those things which are above, where Christ sitteth on the right hand of God.
2) Set your affection on things above, not on things on the earth.
3) For ye are dead, and your life is hid with Christ in God.
4) When Christ, who is our life, shall appear, then shall ye also appear with him in glory.
Those who be "risen with Christ"
shall also appear with Him in glory.
You've written above, "risen in him", I don't mind that statement per se, however, it's not what the passage itself says, and casts a different light from what is actually written.
"Risen with" is translating
synegeirō, "to raise together with". The prefix "syn" means we are sharing the action. I know you know this, I write it for other readers. It's the same idea as in Galatians 2:22, "I am crucified together with Christ", or "I am co-crucified with Christ". We are both together on the cross crucified. It's just what the word means.
This isn't saying we are "risen
in Christ", with your subsequent supposition that we could somehow move into and out of Christ, into and out of resurrection, rather, that Christ was raised from 'among the dead ones', and we were raised with Him.
Here, it's saying we share the action of being raised from among the dead ones, He and I, raised up together. And this having happened, then comes the prophecy, shall ye also be revealed with Him. That same word, "syn", revealed together with Him.
I understand your arguments about what we have in Christ, that "should we stop being 'in Christ', we stop enjoying those benefits", not that I agree for a moment that someone leaves Christ, having been joined to Him. Passages like this one I'm presenting eliminate that possibility.
Having been co-raised with Christ, you are hid with Him in God, and when He appears, you will co-appear in glory.
Much love!