The clock starts running for making the election broadly from when the individual has a new combination of residences. i.e. when you start to use both residences.
I am not convinced that there is a "new combination of residences" because a person changes the use of one of his properties - unless you can point to the specific legislation that allows this.
IMO a new combination of residences can only apply if the individual either sells or acquires a new property such that his/her combination of residences changes and the 2 year time limit runs from that date. Interested if you can point out the error of my ways.
Surely, the legislation refers to when the *residences* change. So changing a property from a residence to a BTL or an empty, unfurnished properly, is a change in *residences* available to the taxpayer.
Indeed, in the past, rented accommodation counted as a residence, and HMRC had a concession waiving the 2 year rule for people who weren't aware that they needed to make a CGT PPR election.
https://www.gov.uk/hmrc-internal-manuals/capital-gains-manual/cg64427