This is a gloriously ridiculous idea, so if it came from the French perhaps its true

The internal arrangements of a submarine are incredibly complex and the sorts of allowances and accommodations necessary to permit a change of this magnitude would be huge.
This is all about the improved capability over a diesel vs. the pacing threat. Throw in some apparently lousy results to date from the deal with France and it was probably a pretty easy call for the Aussies. And it works out great for us because there'll be another dozen Virginia class subs out there, fighting on our side.
jeff