Not sure what you mean with "if I can switch phones if I have IPhone forever" , but once unlocked by the carrier, iPhone's remain unlocked, and can be used on any carrier after being unlocked. They will also not lock to any other carrier.
If your iPhone is locked to Sprint, you will need to have Sprint unlock it before you can use it with Verizon.
https://www.sprint.com/legal/unlocking_policy.html