I don't know the answer, but I know that for my parents transferring properties into a trust, they have only had to have a licensed real estate agent do a valuation with several comparable valuations as back up.
This has been used for the stamp duty and as the base cost for the properties now that they are in the trust.
I don't see why you cannot do the same, as you would be looking for a base cost from when it turns from a PPOR to an IP, wouldn't you?
Of course, you need to get proper advice, but this is all my parents have needed.