Greetings Kramer!
Lots to listen to out here, both analog and digital. To answer your questions:
1. Phoenix has not yet rebanded, and it's not all digital
yet but I'm sure within the next 5 years just about everything worth listening to will be.
2. The Pro-94 will not reband, so I'd recommend getting a scanner that will. You can find a list of scanners that will not reband here:
http://wiki.radioreference.com/index.php/Rebanding#Which_Scanners_Stop_Working.3F so check this to make sure the trunking scanner you buy will be able to reband.
3. The newest digitals will have a simple patch that you download from the internet and apply to the scanner. The Unidens seem to have more of a level of complexity here, there's an ESN number and checksum you need to register with Uniden to receive updates - not sure how that works but I've heard mixed reviews. The GRE's patch process seems to be much simpler, but the GRE scanner is also not as intuitive to program as the Unidens are.
4. Would it be wise to wait? I think now is a great time to take the digital plunge. The Uniden's hardware seems to be designed with evolution in mind, meaning that if you buy a BC396 or 996 today, the next "digital scanner" will be a firmware update you purchase and apply to the scanner you already own. Pretty cool! The GRE's also seem to be designed with this idea in mind but no word yet on whether or not they plan to implement a paid firmware upgrade path. Either way, today's digital scanners are much better than the first generation was, and much more expandable. I recommend the GRE myself (although I have yet to get one) but others here who have them say they're the cat's meow for monitoring the digital stuff out here.
5. Tags? I assume you mean what is the tag all about in the RRDB. I'm kinda hazy on that myself, but the radioreference blog has an article that explains what that is far better than I could:
http://radioreference.blogspot.com/2008/05/as-many-are-aware-back-in-january-of.html
Hope that helps. Lots of helpful folks here in AZ so if you have any other questions ask away!
-AZ