The "F" word.
A long article on the Shuttle foam problems can be found in this NYT article.
This paragraph is the only mention of the formulation change:
NASA engineers had already seen how fixes can break things. After they made a minor change in the foam application process in the late 1990's to comply with environmental rules, small divots of foam rained off of the tank during ascent. The phenomenon, called popcorning, was caused by trapped bubbles; NASA solved the problem by venting the foam with tiny holes, but it was a reminder, if any was needed, that seemingly small changes could have profound effects.
"Freon" is not mentioned.
The executive summary of a report on the subject is here. I haven't scoured the interim reports other than some targeted word searches, but I don't see the subject of the formulation coming up there, either.
Note to NASA:
Get an environmental waiver on the formula. Put the damn Freon back. See what happens.