That makes you a busy man trying to control all those trains at once. Been there, done that.
As for the upgrading; it reallt doesn't change all that much. The decoders have to ability built in. What changes is the ability of the command stations to operate all the different functions that the decoder is capable of.
When I first received my NCE, it could control only 10 functions of the BLI locos and not the momentum either. After some law suit was settles with someone, these guys are always suing each other for some reason beyond my ken, NCE sent me a new upgraded EPROM (the Brains) to install in my Command Station. Two munutes work and now I have 14 functions and momentum and back EMF control. So you see, it is not so much the decoders, but the command stations that need upgrading. That is why I went with NCE which touted easy upgrades. Digitrax and Lenz can also be upgraded, even the starter NCE, but the MRC and Bachmann starters can not, to my reading of the info available.
Your problem will be to convert the many locos you now have to DCC. If you get too many DC locos, you might just price yourself out of the conversion market. A better plan might be to purchase DCC ready locos now and upgrade to a DCC system later. The DCC locos will run on DC in most cases. You just don't get all the features. But, look at the Precision locos that have DC/DCC with or without sound and an optional control box for DC operation which gives you a lot of the DCC features. You have a lot of reading and decision making to do.
B-)