Whether it’s easy to swap out a rechargeable Lithium Ion battery built in to a recent aid very much depends on the model and its design. If Oticons are designed for it in some models, as suggested above, I know that with some ReSound aids, including my Quattros, it is not possible or at least doesn’t make financial sense given how much work would be involved, even for an experienced service tech. The entire unit other than the case must be replaced when the battery is weak, and if you’re out of warranty, aid replacement from the manufacturer may start at $300-$400 per aid, when spare units allow it. Eventually there will be no aids left for replacement for an older model. Remember that in some of these aids, everything is very, very compact and tight, so the battery may not be easily accessed and replaced.
I too remember when anyone could swap out flip phone rechargeable batteries after three years for under 10 bucks and be good to go. Of course manufacturers who prefer planned obsolescence do not look back to that time as being the good old days. This is definitely a potential drawback to rechargeable aids compared to button battery aids.