assuming you are not planning to wait for the mmu2S upgrade,
your mmu2 should have been supplied with a set of extruder housing parts,
build the mk2.5 upgrade using the extruder housing parts from the mmu2 upgrade, instead of the mk2.5 upgrade extruder housing parts, and use the mk2.5 filament sensor cover... check that the printer works well as a mk2.5, then build the mmuv2, change the filament sensor top cap, install the mmuv2, update the printer firmware and the mmu firmware, and start learning how to drive the new beast!
I try to make suggestions that I believe to be well founded and safe,You should understand the context and ensure you are happy that they are safe before attempting to apply my suggestions,
what you do, is YOUR responsibility.