I don't want to sound pessimistic, but it may prove difficult for Mclaren to quick unlock and fine tune the new philosophy its trying to adopt, with the new inboard-loaded fw. Lots of things will have to change on how the aero front to back will be working, and finding the sweet spot on setup may not be so easy in FPs.
What i am saying is that there is a possibility for Mclaren to switch both cars to old fw and aero concept, and wait for Barcelona test to fully understand and unlock the new fw philosophy and aero.
I hope this isn't the case , but i will be really impressed with the team, if they manage to correctly fine tune such a change in aero philosophy using only 3 FPs.