If you are a first time buyer, you remain one (for mortgage relief purposes) for that tax year plus 6 years thereafter. You can change your mortgage as many times as you like and can buy new PPRs as many times as you like and will remain an FTB for TRS purposes.
However, if you buy a different property (PPR or otherwise), you are no longer an FTB for stamp duty purposes.
Yeah I have already a mortgage on my new house, I am wondering If I would be able to move to one of the new fixed rates on offer at the min for first time buyers and also If it would affect my interest relief.
Watersprite has answered the interest relief question - thanks.