Your Old Routes On New Maps issue could be resolved, incidentally, using my guide (I think PCs use MapInstall too?). You can use MapInstall to go back and re-install older maps on your Zumo, if'n you want.
The process is pretty close. I have always used Mapsource which is PC only. I only recently discovered that they had a PC version of Basecamp. While I have installed it, I haven't used it much. I know I have done updates where the routes haven't been a problem. I only mentioned it because it would suck to assume that they were fine and get out on a trip only to discover you couldn't use them. So if you do a map update before a big trip, be sure to test any routes that you may want to use. I was lucky in that the time it happened to me on my Zumo, I was trying to load a route for a trip. I just ran in and recalculated it in Mapsource and tried again and it worked.
As for old maps. I mentioned above that I have multiple mapsets on my Garmins. Both my SP2730 and Zumo 550 have 2009 and 2012.2. Each unit is a little different but you roughly go into settings -> map -> map details I think. Then it shows which maps you have installed. You can enable and disable detailed maps from there. Just a note on this. The detailed maps is the mapset that you install. For instance 2012.2. It includes all the detailed street level views and POI information. If you do a limited load of the map data like you showed in your article and you go outside the area that you loaded, it isn't like you fall off the edge of a map and get nothing. What you will see is only major roads. Basically Interstates, Highways and main county roads for instance and a dot with the name of towns and cities. No small roads or city streets will appear and you can't do address searches or POI lookups outside this area. In my case I have MN, IA, WI, SD, ND, KS, IL, AZ, FL loaded on my Garmins. Now I spend most of my time in the mid west and have family in AZ and FL that I may go see so have those states loaded. Should I want to do a road trip from MN to AZ the GPS would still work as I would likely take just the major roads as I am blasting through the states that I don't have detailed information on. Since I am not retired and have limited vacation I can't take my time by enjoying the back roads on a trip like this. So for me it works. Honestly though, this is more of an issue on the SP2730 as it doesn't have a lot of internal memory. The Zumo has tons of space with the 16GB SD card in it for maps and music.
If you wanted to test this you can go into the settings, maps and details area and disable detailed maps for whatever mapsets you have then go out and look at your screen. This can be turned on/off as you like. I don't see the point of totally disabling it other than testing so see what it is like if you are outside the area you loaded though.