Don't Switch to AT&T if You Don't Want to Buy a New Phone
If you don't like switching phones, you may not want to switch to AT&T right now. While you can bring some phones to the carrier, not all phones will operate at peak levels on AT&T.
When you switch, you keep your number, but you will likely need to buy a new phone unless your phone is compatible with AT&T already. Thankfully AT&T's network works with many phones, so there's a good chance what you have will work, but depending on how old the phone is, it may not have the antenna bands for the fastest speeds.
AT&T will make it easier for you to switch with credits to pay off what you owe on your old phone if you switch and trade in your phone. This will help you get a new phone on AT&T, but you will need to backup and restore your phone, so make sure you have time to do this. It's not going to be a 10-minute switch for most people.