I need to change an .mp4 (h264, AAC-Audio) to an exactly same .m4v. Can I just rename it from "X.mp4" to "X.m4v"? By renaming, I mean clicking on it in Finder (Mac) or Explorer (Win) and renaming it.
So if the video is DRM-protected (i.e. came from the iTunes store) it should stay as m4v (and will open by default in iTunes). If it's not, it can be used with either extension, and many players will be happy either way. By default, .m4v files will open with iTunes, which may be convenient.
Be aware that you are not changing anything. If I call you Bob, you are still Jac and even then you are probably posting under a different name. If you have software that naively trusts the filename instead of examining the actual binary headers of the file, this can work for you. If the actual format or encoding is wrong then no name change will fix that.
It is possible that some embedded systems do rely on the filename to filter a list of possible files and provide no way get around the default filters. Renaming the file can be your solution.
Yes, this simple conversion should work fine. Both files are technically mp4 files anyways.
Notably, certain older Apple products, like older versions of appleTV for example, require it to be in m4v format so don't forget to switch it back if you're switching between a Mac and a PC if you are using older mac products.
By default, m4v files will always open in iTunes vs other programs.