**No** ,it is not possible to change the _facility/severity_ of log messages.
The property replacer document is for regex operations on the log message and not the facility/severity.
The rsyslog sends the packet with its header and log message.The log message can be manipulated with Regex but the header contains the facility and severity which is handled by rsyslog/syslog.
You can pass the _facility/severity_ through log message to remote syslog server,but the syslog server would take _facility/severity_ from header of the packet.