No, REM provides a measure of what the hearing aid is outputting at the eardrum and then gain is adjusted to match targets. If you adjust the gain on your own from there without resetting the fit you are adjusting around the REM adjustments but if you tell the software to refit everything then the REM adjustments are lost and you are back to first-fit until you re-load your saved file.
I wouldn’t generally recommend running the in-situ as it it not the same measure as a traditional audiogram and does not, as some believe, provide a more “true” audiogram*. As to whether running the in-situ will delete the adjustments made when your provider did REM, I think, off the top of my head, that it depends on the manufacturer? The only in-situ I’ve ever found at all beneficial rather than detrimental is widex’, but I recall that running it wipes out previous adjustments (irritatingly) rather than just overlaying changes on top of them.
The tricky bit with self-adjusting is probably considering the biological side rather than just the technological side. The brain adapts to a certain extent given the chance. I recall reading a study, done back when auto-learning in hearing aids was in vogue, that suggested that forcing a patient to live with a certain gain setting for a few months before adjusting to comfort resulted in better longer-term outcomes than adjusting for comfort straight off and then trying to increase them to target (although the latter seems a more popular approach). That is, the former group ended up being comfortable with higher gain and had associated intelligibility benefits. I am sure that I will inevitably be a self-fitter when it comes time for me to wear hearing aids, as any provider probably would be, and I imagine that given the opportunity to adjust for comfort at any time it would be very difficult to force myself to NOT fiddle and simply adapt. Especially given that the biological adaptation is not total, not guaranteed, and we don’t know exactly what its time course is. It would be easier to force yourself to live with a certain setting if you knew precisely how long you needed to do that in order to know that neural adaptation had pretty much reached its limit.
From that perspective, if someone who was a DIY fitter were to go out and have REM done*, it might be a good idea to try to live with the end result for a few months before making any changes. Another thing floating in my brain, and I no longer know exactly where this knowledge comes from which makes it less trust-worthy, is that perception of medium and loud may be more flexible than perception of soft. You can use hearing aids in certain ways to push someone’s perception of “loud” upwards to a certain limit (i.e. they used to feel that 70 dB HL was “loud” and now they feel that 90 dB HL is “loud”, which is more appropriate). You can get used to soft sounds being loud, but unlike medium and loud levels they may never come to feel “soft” the way they did when your hearing was normal, and then you need to make a choice about whether you want to hear something at a level louder than you would ideally prefer, or not hear it at all. Neural coding of softness partially has to do with the number of neurons activated, and with the loss of specificity that comes with hair cell damage you can’t quite get back to it.
As an aside, pediatric hearing aid users often do better with their amplification than those who get hearing aids as adults. In part because their brain is wiring up with the hearing aids, but also in huge part because pediatric prescriptive targets provide quite a bit more gain, and thus more audibility, than adult targets. If you are playing around, flip over to DSL child and see how you do .
At the end of the day, though, this may not matter. If you are self-fitting and you are satisfied with the sound of your hearing aids, does it matter that you may not be optimizing your hearing? If you feel like all of your needs are being met, then perhaps there is no need to force yourself to tolerate a bit more high frequency gain for the sake of getting 88% of conversation instead of 80%.
(*There is some skill in taking a good audiogram, and there is certainly skill in running REM. Presumably, you are working with a good audiogram and good REM settings. I’ll say too that there are little errors that can show up in an audiogram that a provider may be so familiar with that they will adjust the fitting to compensate for what they know is an error in the audiogram, which would perhaps be hard for a self-fitter to do.)