I found the solution to this:
"zmfixreminder" must take care of this. This tool was developed to apply default reminder to an existing appointments, you can check this:
Bug 28633 – Need tool to apply default reminder to existing appointments
It is a command line tool:
/opt/zimbra/libexec/zmfixreminder <options>.
Usage options:
-a <email>. - to fix the named account.
-a all. - to fix all accounts on this server ...