getting my head round switching to BTL mortgage

I currently live in a house worth 120k with a 65k residential mortgage on it

I've seen a house I would like to move to, and would have to pay 150k for it

assuming I transfer my current 65k residential mortgage over to the new property, and rent my current 120k home out on a 75% BTL mortgage... how much shortfall would there be? if any

it's like being back at school this