An observation based on the thorough testing done by Mark McCormick at Aardenburg-Imaging.Com. The OBAs fluorescence effect did not just disappear but a color stain in the paper white appeared as well. Worst paper white shift of any paper measured by Aardenburg if I recall it correctly.
Ernst pretty well summed it up. Perhaps Aardenburg's independently performed testing which revealed such a poor outcome for the original EEF, coinciding with the unfortunate choice of marketing such a non archival paper as both "Exhibition" and "signature worthy" finally convinced Epson management that a serious reformulation of the product was overdue. I'd certainly like to believe this had something to do with it, but other business transactions could have caused it, too. I have no insider knowledge why Epson actually enacted the change to this product.
That said, it's about time. With any luck Epson has resolved the yellow staining problem that went well beyond mere high OBA content loss of fluorescence, and yet once again Epson stumbles by not renaming the new formulation "Exhibition Fiber II" or something similar so that buyers will know the product is "new and improved" (hopefully)
Precedent for acknowledging reformulations exists in many well known product brand names. Fuji, for example, improved the fade resistance of it's original Fuji Crystal Archive chromogenic color paper, and renamed it Crystal Archive II. The roman numeral II on the box clearly delineated the new and improved paper from the older version.
Without a name change or "new and improved" sticker on the box, I will now have to try to identify some of this new stock for another round of light fade testing, but I will still be guessing somewhat whether I"m actually purchasing new or old stock unless the changes to the product are indeed as obvious to me as they were to the OP.
cheers,
Mark
http://ww.aardenburg-imaging.com