Unfortunately the VM administrator password is unique to the template. The password to the local administrator is the same.
To replace it you have to act at the operating system level, with scripts, Laps, etc ...
There is a software called Laps that allows you to set the local administrator user with a random password (never the same)
Local Administrator Password Solution (LAPS):
Microsoft OS: "https://www.microsoft.com/en-us/download/details.aspx?id=46899"
If in your environment I use the Group Policy Microsoft Laps is perfect.
Create a GPO with Laps that changes the password when the VM is first rebooted. I've used it many times and it works well.