Once I get my MAC code, can I still recontract without using the MAC code?
or is it that once you have your MAC code you can't recontract with BT and have to leave?
You can re contract if you want. You don't need to use the MAC code. You may find that after you ask for it you will be contacted by BT retentions team who will try and get you to stay by giving you a discounted offer.
Getting a MAC code doesn't commit you to leave. You can just let it expire if you decide to stick with BT.
Phoning up to ask for a MAC code will put you through to BT's retentions team who will almost certainly make you an offer to persuade you to stay.