Hi Anders
About putting an rendered 24fps sequence into a 25fps comp and retime the already final frames can be problematic on some type of footage. If you want to do a highquality motioncompensated retiming of the footage you will see weird distortion of some overlapping motion, especially the "flying" layers that you use a lot in you animation that moves out of or into the frame, will probably make weird distortion. Here ReVision Twixtor will do a better job than the builtin AE effect. But maybe you can also try with just frameblending and see how it affects your animations, or just try to see how annoying the 24fps straight converted to 25fps will look. maybe it will be ok with your style of animation. Do some tests and make a desicion on that.
What i suggested with putting the original 24fps comps into a 25fps comp, was what you don't wanted to hear. Yes, re-render the original AE animation, that is the only really good way to do it, but might also, as you say, be too much work, and something maybe the target audience wont notice anyway :-(. Again, make some tests.
If you want to have your animations broadcast both in NTSC and PAL countries, i think that it is safer to do what you have done until now, and continue animating in 24fps, since a 24fps converts good to 30fps with some interlacing field doubling (some other US based users might have a better understanding and better description on what to do here), and 25fps have the same problems converting to 24 or 30fps as 24fps has to 25fps.
Only if your mainly works for PAL countries i would suggest to work in 25fps.
Heres a list of what countries has PAL, SECAM (also 25fps) or NTSC: