You will lose TRS on the property once it ceases to be your principal private residence. If you move into your fiancee's house then your house ceases to be your PPR.
Remember even if the house is not rented, then on the next valuation date you will be liable for the NPPR tax.
If you rent the house you will be allowed 75% of the interest as a deduction against rental income provided all tenancies are registered with the PRTB.