http://www.wolfgreenfield.com/files/abrahamsen_patent_reexaminations.pdf

excerpt:

The patentee may, however, attempt to narrow the claims to avoid the cited prior art and, at the same time, try to keep the claims broad enough to cover the accused technology of the challenger. Of course, narrowed claims are enforceable only prospectively from the date a reexamination certificate actually issues, typically several years later, meaning that any damages accumulated prior to that date for infringement of the narrowed claims would effectively be erased.