In article .com,
says...

plumbing suppliers have adapters that can bring you up to new rough-in dimensions.

http://www.quickfixplumbingproducts....0extenders.htm

I didn't want to move the plumbing so cut the hardibacker around
the current flange and use a similar spacer (from Home Despot). My
problem now is that the floor is 7/8" higher than it was and can't
find closet bolts long enough. They have long enough 1/4" bolts
everywhere but they just spin in the flange. The flange wants
5/16" bolts and I can only find them 2-1/4" long. :-(

....one more place to try this afternoon.

--
Keith