I don't think there is a very efficient way to do this, as you would need to remove the domains from all the fields they refer to, add new fields, and create new domains of the correct type referring to the new fields.
Are you extremely sure you need to create subtypes? They are a very specialized thing and are only really appropriate when you have a single feature class than has all the same fields but different domains and defaults (and maybe different topology rules) for different groups. The example I use when teaching is water pipes, where some are trunks, some are mains, some are laterals, they all have the same fields and can be one feature class but each may have a default type and size and maybe different domains as well.
If you have 75 (!) domains i wouldn't do anything in a hurry because it seems to me you have a major geodatabase design challenge on your hands.