In terms of price nothing will change, same price during and after the minimum term with all BT mobile plans (Including handsets), after the minimum term you will continue on a rolling 30 day contract, meaning you can cease the service with 30 days notice or request a PAC code to port the number over to a new network.
The other option is to see what new offers are available, switch to a different plan on a new minimum term, get in touch and ask.
Once a PAC is requested* you'll be given a code starting with BTC
Order your new sim from your new provider, once you have the new sim you can give the new provider the PAC code, this will allow them to port your number to a new provider, once done your BT mobile will cease, the port takes around 48-72 hours.
* PAC codes are valid for 30 days, if unused then the BT mobile will continue.
One last thing is any perks such as BT Sport that come with mobile plans, they stop working once your mobile is ceased or ported