I do this from the regular CPS before we surplus radios. These are the steps I use. I have used this on XTS5000 and XTL5000.
Step 1-Load Advance System Key
Step 2-Read Radio
Step 3-Tools-Password-Read/Write Password, this were you remove code plug passcode if used.
Step 4-Modify Permissions-Uncheck all three boxes, Click OK
Step 5-Tools-Set Owner System ID
Key Type-Advanced System Key
Owner System ID- select correct key number of the system.
Click-Disable Write Protection
Click Set
When this process is done, Close CPS, remove Advanced Key from Computer.
Step 6-Open CPS, Load Soft Keys that match the systems in radio. Read the radio.
Step 7-Add a new system with default key 1, delete old trunked systems and their personalities. Add new Personality for system 1.
Step 8-Add a new conventional system, delete the old conventional system and personality.
Step 9-Delete all the zones except 1, change it to have 1 channel of the new conventional system
Step 10-Make sure you turn off Infinite Key Retention, if radio has secure enabled.
Step 11-Write radio, now is should be conventional only programmed with no Radio System information or Encryption keys. Anyone can program the radios now, for conventional or different trunked system.