First yes the old RX's will work with the new radio.

Second YES you CAN make the trim steps work in the same manner. You will merely have to manually find the setting in the new radio "Trim Steps" page that gives you the same equivalence of the old radio.

When you are going to a higher resolution system, you'll normally find all you have to do is double the number of steps.

I've taken configurations over by hand from one radio to another JR this way, and have not had to perform ANY additional trimming on the new TX.

Otherwise you can merely copy the settings from one TX into the other..