Hello, I have already finished a project with the composition and footages originally set to 25fps. However, I was told later to correct them to 30 fps. I then went back to change all the composition and footages to 30fps. However, this caused all the keyframes to shift and no longer align with the original frames and when I played the composition, I saw images blinked (disappeared?) at some point. Please advise me if there is a way to solve this problem. Thank you in advance for your reply.
AE operates on time not absolute frames, so the behavior is correct - they keyframes are still at the same time they were before, the same time is just sliced into more frames, so a different frame shows up at any given keyframe than before. You will have to manualyl adjust the timing of your keys. There is no way to automatically clean such things up.
You should also check the interpretation on any footage you have in your composition. 25fps video doesn't always play well with 30fps comps.