You just unclip the wing from the backplate and clip on another, it takes less than a minute. The harness however is a bit more fiddly, you have to unthread it and rethread it if you wanted to move it, the better alternative is to buy triglides, d-rings, a buckle and another 12-15ft of strapping and just make one yourself if you have more than one BP. If you plan on diving doubles, chances are you will need an AL BP as steel doubles are heavy enough without you adding more to that. If you dive singles, most people can make up their weight requirement with a combination of SS BP (from 6#-19# - i think i have seen, depending on thickness) a single tank adapter, STA (with weight in the channel of the STA if needed) and other weight in a belt if you still need any or want something ditchable. With my 3mm and an AL80 i wear 1# fresh and 7# salt with my 6# SS BP on a 34# wing (however you can get singles wings from 18-45# i believe, depending on your requirements). This weekend i will try my weighting doing shallow shore dives for sharks teeth in nothing by a 1mm skin or just t-shirt and shorts - will see what that 3mm of neoprene is worth
As for DIR-F, its a course offered by GUE
https://www.gue.com/ that is based around learning the fundamental (what the F stands for) skills of diving, in their system Doing It Right (DIR) see the forum on DIR on this board a little higher up the page than this one.
BTW, you will probably only need a standard BP size, you dont need an extra-long or short one, although what i would call lanky (skinny and tallish), i have 4 inches and 75# on you and i only wear a regular one, but i am probably borderline for the longer plates that are out there and those taller than me are more than likely going to need a longer one. Mine sits roughly a little (1.5-2") above the point of my shoulder blades and goes down to just under my rib cage/approx kidney area - very comfortable like that for me.