PPR

Hi I have a client who owned a property (main residence) and a BTL. They sold their main residence in May and moved into their BTL, which they are now renovating. They are buying a new home which will be their new main residence and they will re let the BTL where they have been living since May. Their solicitor has advised them they wont have to pay the additional 3% stamp duty on the new purchase as they are replacing their main residence. My question is, do I write to HMRC and advise them that their current PPR is the BTL address (in case they ever come to sell and want tax relief for having lived there) or will this impact them getting the reduced rate of stamp duty?