Play A has 1 season left to run on their contract.

If I renegotiate his contract and offer another 1 year contract, will this add to the length of time the player is on the team (ie his contract will now read "2") or will this replace the contract he has?

I suspect it's the latter but wanted to check.

When you renegotiate, the first year of the contract is in the current year, so you will need to sign a 2-year contract to extend it into the next year. You can see how far his contract will go by looking at the table on the bottom of the contract offer screen.

Thanks.