Sorry if this has been answered somewhere, this is my first post here.
If my character has Flexible Formulaic Magic as his major hermetic Virtue and is of house Merinita and has Faerie magic, can I move a formulaic spell I know lateral to a same level faerie range/duration/target as opposed to up or down by one category?
There AFAICS is a problem with the wording of Flexible Formulaic Magic in ArM5 p.42, which is independent of Faerie Magic considerations.
As written, the Virtue does not allow shifts sideways, e. g. D:Concentration to D:Diameter. This does not make much sense, though, and likely was not the intention of the author either. So talk to your SG about this first.
If she, too, agrees that sideways shifts are possible, then sideways shifts to Faerie Magic Ranges, Durations and Targets should be also allowed: after all, all magic cast by your character counts as fay power anyway.