Can I use my old phone on a new contract?

I had verizon, and I've still got my old phone. It's an LG Voyager, kinda rough looking cuz it's taken some falls but it still works. I have at&t right now, an iPhone 3G and my mom is trying to cut off my phone..but my bf said he'll put me on his plan. I'm just wondering if my phone does get cut off, can I use my old phone from verizon on his plan? He has verizon, and idk what plan he has but I need my phone and I'm just wondering if I can use my old phone or if I'll have to buy a new one. Also..would it be possible if I could use my current iPhone with verizon even tho it's the old at&t phone? Jw. Thank you!