If a protestant minister is married (A), he can be a priest when he converts (if accepted) (B)
If a protestant convert priest becomes a widower (A), he cannot remarry and remain in the priesthood (B).
If a catholic priest (A), transfers to another rite (if he is allowed to do so), he can marry and remain a priest (B).
If a catholic priest marries (A), he is no longer allowed to minister (B), but if he dumps his wife (C), he can return to the priesthood (D) (if they will accept him back).
If a married catholic layman wants to enter a seminary and become a priest (A), he will be refused admission (B) unless he divorces his wife and is granted and annulment (C).
That's enough. Now he is still a priest forever, but may not be allowed to function as a priest.