61.75
As far as I know, the EASA licence will have a new number.
I would like this to be the case - keep old CAA licence which is an ICAO licence, keep FAA 61.75 on the basis of CAA licence which wont change and so remains valid on basis of old CAA licence, get EASA licence on basis of old CAA licence which keeps EASA happy and continue to fly EASA types on N reg carrying FAA, CAA and EASA licences to keep everyone happy.
The worse that could happen is that you have to go through the 61.75 validation process again and pay for a new FAA licence - as you may have done for the English language endorsement and the plastic card if your original FAA licence was issued on card.
As I understand, you need an ICAO licence to have an FAA licence issued on the basis of 61.75. Surely the LAPL isn't an ICAO licence so you could not use that for the 61.75.