I have done this a couple of times in the last year to pull flights forward to more "expensive" options when I've allowed plenty of time to make a connection from a *A carrier on a separate ticket, but - as orbitmic says - you must only look on this as an improvement, not a guarantee.

So long as your plans can still work around the booked flight, it's a good option to have. If the cheaper flight doesn't work around all your other arrangements, then I wouldn't take a chance on it unless you're booking very close in and can see the other flights are unlikely to sell out.

Personally I'd probably take drinking at the Beer Festival above drinking in the lounge though, although I guess that's not actually the prime reason for your visit to Munich! However, I doubt you'd find any issue with the duration of your lounge visit if needs must.