You can back up your config, reset (or not) and set up the test config. When done testing, restore from the backup. I believe that passwords don't get backed up, so you will have to re-enter them manually.
If you are keeping the same router and it gets a public IP address on the WAN side in both cases, you shouldn't need to change anything when switching ISPs. However, some routers and modem/routers have quirks that require special settings for SIP, so you may need to adjust accordingly.