If you have a player in his second year of being kept and you trade him...Does the recipient take on those 2 years or his slate wiped clean?
What about only being allowed to keep drafted players. So any trades involving keepers would wipe their keeper status clear and the following year they would go back in the draft.
Having the 2 years of being kept transfer with the player will work, but so will the rule that only drafted players may be kept.
Personally I prefer the first option where the years of being kept transfer with the player traded.