Replace all section breaks (^b) in a mail merged document with page break (^m) doesn't work

Question

I have a mailmerge template to produce invoices. The result of the mail merge is a single word document. I need to split this into the individual invoices documents (1 page per invoice). I had many problems which seemed to hinge around the automatic inclusion of a section break by the mail merge process. If I manually replace all of the section breaks with page break all is OK. The problem is that FIND ^b (section break) finds all section breaks correctly. REPLACE ^b with ^m (page break - or anything else for that matter) returns with zero replaced