I did a quick search and came up empty.

What's the rule on Shapeshifter characters who have an Increase Attribute sustained on them in their lower-attribute form, and then shift to their higher-attribute form? Does the original bonus apply, do you have to record the # of successes for the higher attribute, or does the spell simply fail?

any official word? General consensus? Clever house-rulings?

I'd say that it dispelled.
OR
on casting your target stat number is the higher of the two forms. then there is a static +# to eather form

Well, by a close extrapolation of the RAW...
Whatever screws them the most. Canon doesn't like PC shapeshifters.

Since you know the character has 2 sets of stats, I would go with recording the # of successes for each set.