I can't speak specifically of migrating E-Commerce to Ubercart, but with migration you have a four stage process really:

• Understanding your Data
• Getting your data into a common format
• Migrating/moving/transferring your data
• Testing/auditing the results

This tutorial is good for giving an overview of migration to drupal, but as you already have the data in a common format (your MySql database) that is one of the stages already completed. http://www.lullabot.com/articles/drupal-data-imports-migrate-and-table-w...