How to you tell your sensei (with respect) their waza isn't good any more, that it has deteriorated and become stagnate. ...
Is this from Gozo Shioda's diary? He said similar about Ueshiba post-war. What did he do? Ask for 9th dan, while running his own school! There you go.
ps I think Shioda was wrong about Ueshiba, and maybe you are wrong about your teacher. As has been pointed out, people who are actively practicing don't normally get worse, they normally get better. Sometimes that means they are starting to prefer something that you don't.
If you ask her, "what do you think about this technique" and then do your superior "old way," maybe a good conversation could be had.
If they have a neurodegenerative disease, then you can't get them to "regain the lost skill," you can only support them.