Well, I'm done. My multi-portal production instance simply would not upgrade. I built a virgin 7.0.6 instance of 'Awesome Cycles' and upgraded it to 7.3.4 without issue. I then added all the skins and modules, upgrading where necessary. I then exported portals [as templates, w/data] from the old site and imported those portals to the new 7.3.4 instance. Active Forums bombed 'benevolently' and you need to start anew with any AFs [or simply not use it again; it should behave better. I'm thinking SMF is a better choice and decouples the Forums]. Links Module content gets lost. [a big deal; won't use Links Module again either, and I replaced all with HTML Modules...]. Events Module data all gets lost [I don't get it... why?? I may look for a decoupled community calendar package, not within DNN]. Luckily we're at the beginning of the year and 2015 events were largely not yet entered. Losing all recurring events from past years was a bummer, but a small price to pay in this case for a solid site with all security fixes. All users will have to re-register. Not that big a deal on those sites, though.
Overall, as Sebastian advised, not a good plan for an upgrade...
My other sites with many, many users are essentially single-domain sites. None use Active Forums or 'unusual' modules. I am hoping they will upgrade like the virgin site did; seamlessly. One uses the Events Module heavily and I cannot afford a thousand angry users and rebuilding the events calendars. More on that later.
Is there some place I can go, or a training module I can watch, to help me understand what is going on in my web.config files? I have several different working 7.0.6 instances and the web.config files seem drastically different. Sometimes my web.config seems to be updated by the app itself; at least the date changes... What modules can change the web.config, and why? I never see my spreadsheets revising the Excel executable...
Bob