This is the official text from the EU site:
The Withdrawal Agreement guarantees these citizens and their family members broadly the same rights as they have now: they can continue to live, study, work and travel freely between the UK and the EU.
The same applies to any EU citizen who moves to the UK or UK nationals who move to an EU Member State during the transition period.
Who is protected by the Withdrawal Agreement?
The Withdrawal Agreement protects those EU citizens residing in the United Kingdom, and UK nationals residing in one of the 27 EU Member States at the end of the transition period.
It also protects the family members that are granted rights under EU law (such as current spouses and registered partners, parents, grandparents, children, grandchildren and a person in an existing durable relationship) to join their family member in the future.
I only found all this quite recently and for me appears to be perfect! Most of the French online system is aimed at UK residents wanting to move to France. I am a UK citizen, permanently resident in SPAIN wanting to move to FRANCE. My Spanish residency entitles me already to all the rights of an EU citizen/resident and will continue to do so after the end of the year… As I understand it