I have just had a long discussion with my accountant and her comments to me may be helpful.
Now bear in mind we have lived in France since 1999, I am 76, married in France, and always paid our taxes - the latter being very significant.
She said that Brexit will have 3 impacts on us:
a) Access to the Health system is governed by a bilateral treaty between the UK and France. So it is not a function of EU membership. That is not to say it cannot change but that is obviously an unknowable future.
b) The tax system is also governed by a bilateral treaty and is not a function of EU membership. She also said that she has clients who are non-EU citizens (Brazil was the example) where the treaty between Brazil and France is to all intents and purposes the same as the UK. In other words that will probably not change.
c) Inheritance Law. That is governed by an EU treaty and thus we may end up having our estates managed under French inheritance law again. In our case as we married in France under "Communate" - I think that is the right phrase, then the survivor gets all the property and assets.
She went on to say that the Carte de Sejour EU was not necessary as it added nothing to our right to stay in France an dwas only useful as a means of identity. Since I have a French driving licence plus a very old Carte Sejour that expired in 2005, I need do nothing. I say that as I have been stopped by the police - usual road side check - and they were quite happy with my identity (even though the photo was taken 17 years ago!)
She said she will be writing to all the local prefectures, Gironds, Dordogne, etc seeking clarification and will write up a newsletter for her clients in due course when she has more information. I will ask her consent to share this information with you.
So overall, her advice is do nothing it is far too early.
Last point. She said I would be OK but anyone who has been playing games with the tax system may well hit major problems post Brexit. Good advice as always don't cheat you will eventually get caught.
Hope this helps.