Q: if I buy an ipad retina with verizon, can I switch to at&t later if i get their plan

If you get either the ipad or ipad mini on a contract with Verizon it will be locked to theyre network only, once the contract ends you can then request a unlock with verizon, they then contact apple & the unlock is done via the apple app you have on youre PC. Verizon should explain the process  at the time of the request for an unlock..If you buy a unlocked or sim free version then you can swap and change networks as often as you like by just simply using the sim on which ever network you wish to use..hope this helps