You backup your database before any major changes are made.
You write down passwords three times and tape one to the ceiling.
There is a utility that says it recovers FMP passwords (or resets one user to no password) $29 ??
I had several old databases. I used Passware Kit-Professional. It removes the passwords and it shows all User ID that have been set up. I have used it on *.fp7 and *.fmp12. But it is for Windows. I did not find a program for Macs to do it.
first thing on new dbs I set a Full Access account with my first name and 'generic' password
doesn't help you now, but future...