This was the last time the specific subject was discussed in regards to FFM, and from this discussion the current errata was issued (I'm bringing us directly to the final consensus, but I reccomend that those interested read the whole thing. Several people involved in the current discussion were also vocal in the previous one).
FFM being able to change size is, for any purposes, the intention. And this is supported by the current errata, already quoted by Callen, which I repeat below:
Targets and Sizes (p. 113): Add the following paragraphs at the end of the insert:
"Each Target level includes all of the possible sizes of that target, even though the spell level changes. The size of the target can be manipulated in the same way as the Target parameter, whether through Virtues,(...)
While I don't think anyone is really arguing about that at this point, I just wanted to clarify if this is where we are all standing. Let's try to move forward from here.
The above said, Individual is Individual, and Individual and Group are two steps apart. Individual +1 Size and Group are 1 magnitude apart, but it's not clear that they are 1 step apart. Personally I am on the field of 2 steps apart, regardless of size modifiers. My reading is that this is also consoant with the errata, when it states
Each Target level includes all of the possible sizes of that target, even though the spell level changes.
and later
but these are all changes within the same type of Target.
To me this is clear indication that Individual and Individual +1 are both Individual, regardless of the size difference. This seems to be also David's intent, judging by his initial proposal of errata (which I copy below), but I have been wrong before about interpreting other people's intentions, so take it with a grain of salt:
Magnitudes for greater size are a part of the Target parameter, and can be manipulated in the same way as the Target parameter, whether through Virtues, MuVi, or other techniques. However, changing the size does not change the Target parameter.
Anything different from that requires a clarification to the peripheral code a clarification regarding rules intent, possibly resulting in a change to the Errata. An addition regarding the particularities of FFM could also very well be necessary.