But AFAIK it can be uninstalled with apt ie 'apt-get remove webmin'. I would definately uninstall the TKL version first (and unistalling webmin should also uninstall all the separate modules.

Note though that the TKLBAM module may or may not be compatible. Once you have installed the new version you could try installing 'webmin-tklbam' and see if it still works.

Also not sure if it's still the same but when I last installed Webmin from thier upstream repo (admittedly a long time ago - IIRC it was into Debain Lenny) it was a single installer which includes heaps of modules that you may not want (and refer to software that you may not even have installed). So junks up the interface with menus and menu items that aren't relevant. Any other (non-'core') modules you want installed have (had?) to be installed in quite a painful way (they were tar.gz archives). Although in fairness perhaps that's been improved?

I doubt that the TKL devs will use the 3rd party upstream repo for both of the above reasons (or at least the first if the second has been improved). To risk updates breaking the TKLBAM webmin functionality would be a risk I couldn't imagine the devs being willing to take. It would be good if it were packaged for Debain though and was in the Debian repos. They'd use that I'm sure. IIRC it has been in the past but was a bit of a mess...

Also one other note, not sure how upstream installed Webmin will react with the automated updates when installed from their repo. Ages ago when TKLBAM Webmin still insluded the in-app-updater there was a situation where if you allowed Webmin to update itself it would update fine, but then if you did and apt-get upgrade it would revert to the old version again. I guess that would be resolved by upstream also refreshing the package in the repo at the same time as they allowed the in-app upgrader to upgrade? In theory that should work I guess...

In my random travels I just found a post that may be relevant to you trying to upgrade Webmin. This post is about a year old, but I suspect that it still applies. Perhaps give that a go? Let us know if that works for you...