if the messages that I really want to see are in trace level, then what?

You'd have to restart your application, or instrument it with Byteman.

This was proven to be a performance problem, we can't revert it. Maybe it's not an issue anymore, but I'd need to spend a lot of time to make sure.

An alternative solution would be:

• we use TRACE as any normal level, so it doesn't surprise you
• I remove the TRACE statements from the source code which are performance-wise troublesome

At that point I can expect you to ask

if the messages that I really want to see have been deleted from the sources! then what?

And my answer would be: checkout the source code, add them back in, rebuild, and redeploy :-/