The long-awaited 6th and final part of the excellent British in Europe Withdrawal Agreement explainers, which are fast becoming the go-to source general info (as the actual WA is written in dense legalese).
For details of permit exchange we are of course waiting on the Greek legislation so no-one knows anything yet…no point in going to ask…they don’t know either!
If you have less than 5 years residence in your host state you can be absent for a maximum of 6 months per year to fulfil the conditions of residence. If you have permanent residence as an EU citizen (ie. more than 5 years of legal residence) new rules in the WA will allow you to be away from your state for 5 years.
Upgrade those beige permits if you have more than 5 years on them!BiE-WA-p6-FAQs
direct source – FB message of British in Greece