Players who already have a club can move to a different one through transfers.
As a reminder, signing a player to a contract does not amount to transferring ownership of that asset to another party. Clubs simply hire players for their services for a limited duration, while Agents retain the economic rights to players. Because of this, and because of how contracts work, transfers will initially be akin to trades.
Much like in the NBA, you can think of it as clubs trading contracts rather than players. The number of contracts traded does not have to match. For example, a club can trade three player contracts in exchange for a single one.
Trading players has ramifications for their Agents, who may disagree and may not want to see their players make the move. Agents, therefore, have the power to veto trades. The trade is made if both clubs, as well as all of the Agents for the players involved in it, agree. Once the deal is through, the club's information is replaced on player contracts. All of the provisions agreed upon at the time the contract was signed, however, remain unchanged. That includes both the duration of the contract and the remuneration package.
We intend to eventually make it possible for clubs to transfer players in exchange for rewards rather than contracts. We are also planning to add transfer clauses, which would allow clubs to offer extra incentives in transactions.