I have a bunch of templates sitting around that have comps that are 30fps. None of it is live motion video, only AE text and shape animation and so on. I import these AE projects into my master project, then drag the animation comps into my master comp for putting something on the top of it. There's a framerate mismatch since my master comp is 24 and the animation comps are 30. I usually deliver at 24fps, so would it be wise to go in and change every single comp to 24fps? I don't know what the effect would be of leaving them as 30 and exporting as 24, compared to actually having literally everything at 24.
If the timing is critical then drop your existing comps in a new comp with the desired frame rate. Everything except the footage will properly adjust so you don't get frame blending. If there is video footage you may get frame blending.
If timing is not critical just copy your original comps and change the frame rate. There is one possible problem with designing a comp originally at 30 fps and then delivering at 24. 24, or for video 23.976 is much more susceptible to critical panning speed judder and other stroboscopic motion artifacts than comps at 30 or 29.97. Rolling titles or graphics (vertical movement) that works perfectly and is readable at 30 or 29.97 will probably not be readable at all at 24 or 23.976. If there is a lot of movement, especially slower movement, in your compositions you'll have to check the projects with a test render. Check out this article from the FAQ section of this forum if you have problems: FAQ: Why does horizontal motion stutter (judder) in my movies, such as during pans?
One other note. You need to be knowledgeable about frame rates and delivery specifications even on the internet. Know the standards, use them, and don't just deliver at 24 fps because you think that's what you want, make sure that's what will playback the way you expect.