But you can read all about how to do deco dives. It is just putting a regulator in your mouth and breathing after all. And taking one out and putting another in is covered in the initial training. Seems like the concequences of screwing up using a long hose are remarkably similar to screwing up a deco dive, worse perhaps.Oh, I don't know, maybe something about deco and trimix (and twins to an extent) being part of technical diving, you know, where it's fairly easy to die if you screw up. I have no idea why you are comparing that sort of training with switching the length of your regulator hose in open water recreational settings.
Nobody needs a class for a pony either, BTW.
You are ignoring that each of these is a small step beyond the previous step. Anyone that can do a dive really can do a deco dive, anyone that can do a deco dive really can do accelerated deco and anyone that can do accelerated deco can do Trimix. None of those steps is fundamentally harder than going from a 40 inch secondary to a 7ft primary and bungeed secondary. So if you want to propose doing that based on what some random internet poster tells you then why not the whole deal?