If I replace my SIM card with one phone to my older phone what happens?

Ok, well actually removing the sim card only takes 4g away on your phone. The sim card is just for 4g signal, it doesn't act like a regular sim card which your number and info is in. So this is what you can do. Since you do have a micro sd card which came with the phone, if you want to get all of your contacts on your other phone, then you can transfer all your contacts to the sd card. Now unfortunately, you will have to go to the verizon store and unactivate your thunderbolt, then also reactivate your old phone, and then all tthe problems will be solved. Good luck