We wouldn't do an ET410 to ET410.2 upgrade kit, we'd just release a .2 kit. Same thing for SC, we would just release a whole kit. I agree it would be a nice option to have though.
Most rc companies don't do "upgrade" or "update" kits anymore. Unfortunately they don't sell well, most customers just buy the newer kit. And the ones that don't buy the new kit either upgrade as their parts wear out or buy one or two items they think are most necessary. If it were financially feasible I'm sure we (as well as other companies) would do it.