One way to do this is to alias the field names so they don't change.
Duane Hookom has an example:
   dynamic monthly crosstab reports
at:
   http://www.access.hookom.net/DynamicMthlyCrosstabRpt.htm

If you want to do it programmatically, you will need to be familiar with VBA
code. Set up the report with the maximum number of text boxes you could
need. Name them txt0, txt1, etc, and leave them unbound. In the Open event
of the report, OpenRecordset on a Totals query that will let you determine
the field names you need, and assign them to the ControlSource of the
unbound text boxes. At the same time, generate the list to include in the
PIVOT clause, and assign the entire SQL statement to the RecordSource of the
report.