It is really important that you inform your existing buy to let lender that you have moved into the rental property. You could be risking mortgage fraud if you don’t.
If you intend to stay in the property you will need to switch to a home-owner mortgage which will be based on your personal income and credit worthiness.
If you intend to buy a new property as your home and re-let the rental property, then you could either stay on the existing buy to let mortgage if the reversion rate is acceptable, or look at remortgaging onto a better rate if it makes more financial sense.
1st January 0001