Doesn't matter what you have decided, the decision isn't yours. BT will at some point move your BTYahoo email to the BT mail system. You have no say in when that will occur.
I’ve decided that the status quo is satisfactory and not to change. There are more important things in life.
As has already been said, it is not your decision. You will not be able to remain on the BTYahoo system. The system is being closed down and if your account is not migrated, albeit that is not an option, it would result in all its folders and contents being irrevocably deleted.