I built an animation using After Effects in the following format: HDTV 1080 29.97, 1920x1080 square pixels 16:9. I have now been told that the project my animation is going into was shot in PAL and is being edited in 25p.
Canmy animation be converted to 25p, or does it need to be rebuilt and rendered at 25p to be usable? Is the pixel ratio (1920x1080) the same for both formats?
Yes, it can - just create a PAL comp, drag your 1080p comp into it and then scale it down. AE "thinks" in seconds, so it doesn't really matter that you're comverting from 29.97 frames/sec to 25 - it'll reinterpret the keyframes on the fly.
Regarding aspect ratios, it depends: is the PAL destination widescreen or 4:3? Adobe changed the default aspect ratios in CS4, so a 1920x1080 comp dragged into a 16:9 PAL comp will either have black bars either side, or crop a small amount off the top and bottom. It's up to you which of these is more acceptable.