The LAPL(A) to PPL(A) upgrade course comprises at least 10 hours flight instruction of which at least 4 hours is supervised solo, including at least 2 hours of solo cross country flight time with at least 1 cross-country of 150nm landing at two airfields different to the airfield of departure.
In other words, you will not save anything by doing the LAPL first and then upgrading as you will ultimately have to complete at least 45 hours flight instruction in either case.