Fixing your data source is the only solution. You have not mentioned what
your data source is so it is difficult to advise the best next step, but it
would be simple to do in Excel, and relatively simple to do in Word using a
wildcard replace - see http://www.gmayor.com/replace_using_wildcards.htm 
where you will find an example.

You will not be able to do it during the mail merge operation.  The the
article "Finding and replacing characters using wildcards" at:

http://www.word.mvps.org/FAQs/General/UsingWildcards.htm

does however contain a section on how to reverse such information.

You may need to use a catalog (or in Word XP and later it is called
"directory") type mail merge to bring the information into Word so that you
can change the order and then use that file as the data source.  In the
catalog or directory type mailmerge main document, insert the merge fields
into the cells of a one row table, execute the merge to a new document,
execute the Wildcard Replace, then insert a row at the top of the table into
which you insert names to be used as field names in your mailmerge and then
save that file and use it as the data source.