Can I transfer just the remaining minutes from another phone onto a different phone with an existing account while retaining its number or does this procedure work only on a new phone? I don't want to lose the existing minutes or telephone number on this phone in an experiment gone wrong.
You were right to be cautious.
I don't think you can transfer minutes between active phones. You can only transfer minutes when transferring service from a current active device to a new, unactivated device.