You're saying (I think) that you have existing XML that's non-conforming because it hasn't already used entities for the 3 required characters in the data...

  Yes! That's exactly my problem. The creation of the XML is not under my control, which drops this problem in my lap.
  As a sidenote, I should probably tell the source system that they shouldn't call this XML.

  Thanks for your replies. I was hoping to do a simple, fast script that would rip through the data (~150MB) and escape out the reserved chars.
  I'm going to look at your suggestion of parsing the data. Even though it might take longer, and/or more overhead, it is a good way of insuring I'm doing the right thing.
Thanks!