Page 3 of the corrected advanced rules from the Splotter website say:
"To prevent others from taking over your contracts, it is allowed to put an earlier year-marker on a contract you have claimed yourself."
Does this take a turn or not? That is to say, let's say Player A previously claimed a contract earlier in the game with an end year of 1592. Then let's say Player B spends an action on his turn to take over that contract by proposing an end year of 1590. Can Player A right then and there block Player B by putting 1589 on the contract? Alternatively, does this rule simply mean that Player A can spend an action on his own turn to change the year that he previously proposed on a contract when he claimed it? I think I'll play by the latter interpretation unless someone suggests otherwise.
I am sure that your latter interpretation is correct, i.e., a player must spend an action on his own turn to change the year that he previously proposed on a contract when he originally claimed it. There is no specific sentence in this rules section saying that the original claimer gets to do this via a "free action", either during his own turn or another player's turn during Phase 3. Essentially, the player is "taking out a new contract" by altering the terms of the original.