A Satan worshiper can change his mind, true, but taking the mark of the beast is something clearly unique. I don't believe they will be capable of repenting any more than Satan and his fallen angels can. Whether it's because they can't or they won't, the end result is still the same. Some spiritual lines once crossed can't be uncrossed. In any case, I sure wouldn't want to be the one to put your theory to the test.
I think it can be inferred from Scripture that the Mark of the Beast is tied to worshipping the Beast. They are often mentioned together as to be synonymous:
Rev 14:9 And the third angel followed them, saying with a loud voice, If any man worship the beast and his image, and receive his mark in his forehead, or in his hand,
Rev 14:11 And the smoke of their torment ascendeth up for ever and ever: and they have no rest day nor night, who worship the beast and his image, and whosoever receiveth the mark of his name.
Rev 16:2 And the first went, and poured out his vial upon the earth; and there fell a noisome and grievous sore upon the men which had the mark of the beast, and upon them which worshipped his image.