I looked into upgrading a DR400 engine many years ago. The CAA wanted irrefutable evidence that there would be no difference between the original 180 hp aircraft and the aircraft modified to 180. Robin would have to be very cooperative, and the cost would be high. The only way I think you could make it work would be to get Apex to do the work and certify that your aircraft was now a full specification 180….
On a PFA permit, provided the larger engine installation is PFA approved, then you will have very little problem. A large number of 80 hp Rotax 912’s have been upgraded to 100 hp at a mod cost of £22.50 and a signature from an inspector.
Rod1