So I think that you are trying to satisfy Q3, is that correct?
In which case if you see the vendor partition you satisfy Q3. (Many ROMs one can only do this with payload bin … other methods to check presence of vendor are satisfactory.)
However now that I see (apologies) that you plan official to community this will / should involve change of build keys so that is expected not “correct”. So I think it will be fresh install.
Dirty FP4 upgrade community was a success following from Week 24, Development and Testing Updates - #39 by aibd