Hi,
As you will have two properties, you will need to confirm in writing, the date on which you consider the second property to be your main residence. Please see the section 'only or main residence' in the guidance here:
HS283 Private Residence Relief (2023)
When calculating your Private Residence Rlief once you have disposed of the first property, you would add 9 months to the number of months the property was your main residence over the total number of months that you owned the property. This fraction is used to work out the available tax relief against any capital gain arising from the disposal. Please see the guidance above for more information.
Thank you.