When trying a different output format for my log files I noticed that LogViewPlus was locked on the old definition from the previous format used. I assume it's pretty easy to change the default configuration for .log.

But what if I have more than one system I need to monitor and the other system is using another format for .log?
How do I handle that?

It would be best to leave the default configuration for *.log.  Instead, you should add a parser configuration according to the full log file name.  Please see the Log Parsers documentation for more information.

It is possible to have multiple configurations for the same target file pattern, but this approach would not be recommend for a generic file name pattern.