Katherine, IF you don't have many records in issue table(for example, not a million records), you could
--run
pdm_extract -f "select id, zCustomer_Zipcode from Issue" > mydata.txt
--go ahead change zCustomer_Zipcode data type in SDM and database
--run
pdm_extract -u -f mydata.txt
However, if you have too many records, you would better do this from database side and your DBA can assist you with this. Doing pdm_load with a million records will take too long time.
Thanks _Chi