[Ziggy1999]

I have been really enjoying the old flipnote quality roms emulated on my homebrew wii. Ones like the pokemon volumes and the Cartoon Network collection bring happiness allong with the nostalgia. Although, it seems modification of these roms seem non-existant online. It’s really a shame and i would like to possibly make fan made volumes of these glorious roms if possible. Video/audio compression is easy enough but how would I, or anyone go about swapping videos and whatever else within these unique roms. Thanks to any help i can get, as i am new to rom hacking. 

[FAST6191]

You don't. The GBA is a fairly weak device when all is said and done (a 16MHz ARM7, with less than 300KB of RAM and storage which typically tops out at 32 megabytes*) so its video decoding capabilities are very limited. Nobody has yet truly bothered to fully reverse engineer the GBA Video series format (even the DS ones are spotty).

*there are a couple of GBA video entries which go higher, most will be said 32 megs -- https://mgba.io/2015/10/20/dumping-the-undumped/ for the more technical stuff there.

If you desperately want videos on your GBA there are some homebrew options. The main one people will look at is meteo (hold out for Meteo 1.4.0 if you can -- older versions are floating around and higher numbers actually means something here), I can't give a link as it is technically a paid for program. It is a video program of the era so make sure you have VFW codecs installed for whatever you are decoding (I always liked ffdshow), or first render in uncompressed or something back.
After that there are no "click and go" type options. https://gbatemp.net/threads/video-codec-for-gba.354591/ being a nice discussion of other things that went. There was a slideshow program that played back music but we can probably skip that one.
There are also the "PlayYan" (Nintendo's own take on a media player for the GBA, not sure it ever made it out of Japan) and gbamp (a flash cart, also known as the GBA media player) but both of those are hardware options and will not work with modern flash carts.

The only "hacks" I have seen are to disable the GB player check (for quality and probably copyright reasons it was locked out of the GB player)
https://www.caitsith2.com/gba_video_arv3.htm

[Ziggy1999]

Thanks! Ill look into these programs but it does seem like a hassle just to modify those poor quality video roms. I was thinking of just replacing one of the video files with one of the same format and file size/run time and seeing what happens, as getting it to actually work is the first step. Making my own gba video rom from scratch may be my best bet if anything if i had to guess .