It depends, you need to read the fine print details so you know what to expect. Sometimes, trading phones in is kind of like “gambling”, sometimes they make an excuse to why your phone isn’t accepted, I’ve heard many horror stories about it, there were people who sent their phone in and never got any money and never got their phone back because Samsung said those phones didn’t qualify or something. So I suggest you research first, maybe ask them if it’s okay or not.
If your carrier accepts it, then I think you’ll be fine. The problem is most of these carriers and companies like Apple often use a 3rd party to handle trade ins, in some cases these 3rd party companies can be a bit shady.