I'm in the middle of doing one of these jobs myself - but that's because a MTM shirt was accidentally remade with plastic rather than MOP buttons, so I'm cutting the MOP off the "first try" and putting them on myself rather than hassle the vendor again. Unless you're a much better/faster sewer than I am, it's going to take a good little while to do all that sewing!
Think hard about why you want MOP - some varieties are prone to chip or crack, especially the super-thin ones. To be honest, though, with the right cleaners for shirt laundry (or doing it yourself) it can work out just fine. Personally I do prefer them, but especially on an old OCBD, they seem... unnecessary?
Bottom line, even if you thrift MOP-button shirts (which is actually quite easy) there's the minor cost of the shirt - but the major cost of the time. You'll have something like 26-30 buttons to remove, then 13-15 to sew back on (to the other shirt) - to do it right, securely and looking good, that's some pretty significant time spent, as I'm finding out.
Doing it for a whole closet? Maaaaan, that makes my fingers hurt just thinking about it. But hey, if you don't mind the time sink and don't mind the (admittedly minimal, assuming thrifting) expense, and like the style/way it looks... heck, go for it!