Thank you for your question and for asking for me.
You will need to ask the Mortgage Company to change the mortgage to a Buy to Let one - and pay the extra Interest involved
Please ask if you need some further information first
There is risk that notifying your mortgage company will indeed mean that they call upon you to redeem the mortgage.
However it you are already considering an alternative buy to let mortgage then this will not be a problem
The lender will not do that - they simply want the money paid - so no problems if you are redeeming the mortgage!