Flexible Formulaic Magic

When I look at the advantage as written, it only allows you to adjust the difficulty of the spell by a little, which then has the effect of changing Range, Duration or Target by one step. So, if you had a spell which had Diameter duration, you could drop it to Momentary or raise it to Sun or raise it to Ring, but you couldn't change it to a Concentration duration spell. Similarly, if you had a Eye range spell, you could change it to a Personal range spell or a Voice range spell, but not into a Touch range spell.

Does that make sense? Why would the advantage work that way?

As the virtue allows you to add or remove a step in mag, I'd consider it normal to allow a change to a different RD or T at the same mag. Seems odd to not allow it.

Allowing just this appears to be a very common house rule.
Probably because it makes more sense that way.