Yes, you should always have the REM redone after switching types of domes.
Your example makes me wonder if Costco has a long term sustainable strategy on offering new KS aids every 1 1/2 years or so, with a reduction of $100 each time. I believe the KS6’s are ReSound, and they make good aids. I see they are MFi so will still be fine with iPhones. ReSound has a name for having good apps for their phones, so not sure if you can run the latest app or not, but apps are more for bells and whistles rather than functionality.
I would kind of wonder if there really was a step up in performance from the KS6’s to the KS7’s which are Rexton. Don’t know enough about each of them to really comment. There certainly was a step up in the KS7’s to KS8’s, which are both Rextons. Now again, I’m not so sure the KS9 is a step up from the KS8. I had an option of making the switch and decided not to.
I think people in general want to think that as the years tick by and the KS model numbers tick up they should be seeing a step up each year in performance. Not sure if Costco can keep up the $100 reduction trick. They certainly hold a lot of market power, so perhaps…