Q: Does changing the sim card delete contacts?

Once upon a time, a long time ago, contacts were stored in the SIM. But not in this century. All your stuff resides in the phone's storage area, whichever size you purchased.

However, now that you have awareness of the importance of all the stuff in there, learn how to back up the device. See About backups in iCloud and iTunes - Apple Support Personally, I prefer using iTunes onto my Mac's hard drive, much faster than having to depend on the network.