Of course nothing would happen. And if it would scare some people, change your cruise CG to 25%.
For the ones that never get scared, change it to all the way fwd, now you are flying in the yellow band, one way or the other. But apart from that, no, nothing happens..If you can get there in the first place, you can stay there..
Again, how exactly is your weight today? How correct is your load sheet? And now you have your engine degradation of a few percent. Your flight plan takes it into consideration for fuel calculation, but your FMC has still the same old database.
This database determines your so called Cruise Trust Limit.
All this leads me to select MCT as a limit, when I deem appropriate, as Pilot In Command.
And that was this topic all about.