Convert Mac Pro internal boot drive into an external harddrive without wiping
You don't even need to move it to another slot. Install your SSD, set it up the way you want, then go to System Preferences -> Startup Disk and choose the SSD.

This isn't Windows, and it isn't 1993. You can have as many bootable volumes online as you have disk space for, Macs boot the one you want booted.