TSM if you use an engineer to do the work on your aircraft then the cost difference between a PFA Permit and a Private C of A is insignificant. However, if you can do the work yourself (with the assistance of a friendly engineer if necessary) then you can reduce your costs significantly. It also removes the future requirement to have your aircraft at an M3 licensed establishment to do the Star Annual.
Personally I'd be glad to have my Auster on a PFA Permit as my Cub used to be. If nothing else it would probably increase the value as Austers have been traditionally seen as "expensive" to maintain because they weren't on a Permit.
I would not want to be on a C of A Permit as I have heard that this is onerously expensive but have no details as to why this should be.
What is the timescale for this changeover? Have you heard anything from the CAA?