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.
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.
Only notable thing is that 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 your 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.