Mistine, up there in the discussion, you have a lot of questuions concerning servos. Most all servos are interchangable with some small amount amount of applied X-acto technology. Keep the black (brown) to black and red to red. With old Airtronics you need to lift the brass restraints and slide out the black and red and change their positions. They are then the same.
One other item if you are relatively new and have not accumulated a bunch of radio gear. Some radios, like Futaba (Tower) and standard Hitec have what is called "Negative" shift whereas JR and Airtronics (Sanwa) have positive shift. The transmitters and receivers don't like each other and don't speak socially.
Now Airtronics and Hitec have a couple transmitters that have programable "Shift Select". JR doesn't provide that. I do not know about Futaba. Hitec also makes some selectable receivers. I like 'em. There is some new and better stuff coming on the market.
My main point is that it is a bad thing and I do remember it well when a transmitter goes bad, and you send it in for repair, and there you sit with two nice models to fly, each ready to go, but only one can go because one cannot be flown because the one transmitter can't talk to it. It's a bunch of trouble to be changing receivers at a flying field. [:'(]
Just my thoughts as I read your thoughts about having a JR and thinking about a Tower, well I would think about compatiable equpment for the time being. If nothing else, those Hitec receivers are as good as money can buy and just switch a plug around and any transmitter is good to go. I have about 15 of the little critters.