Data Migration is upgrading from one version to another version fro e.g. from 11.5.9 to 11.5.10.. Data Conversion is receiving data from a legacy system or receiving data from another module ( e.g. PA), validating it and converting it to ur our the target module using the standard import program.
Data migration means moving the data from 1 particular version to another for e.g. migrating data from tally 9 to tally E.R.P 9 and data conversion means converting the format of data for e.g. converting video from avi format to 3gp