CGT and any other implications if property is used rent-free

A relative has lived in and owned as their main residence a small property for more than 15 years. The relative has decided to move out and commercially rent a larger property. I have been told that I can live in the small property rent-free. Does this arrangement change the designation “main residence” of the property for my relative? What are the CGT implications of the small property if it is not sold within 9 months of my relative moving out? Are there any other financial implications to this arrangement which enables me to live rent-free.

Hi,
Please see guidance here:  
HS283 Private Residence Relief (2024)
Thank you.