I'm fully redesigning a site that indexes a number of articles with basic search functionality. The previous site was written about a decade ago, and I'm salvaging about 30,000 entries with data stored in less-than-ideal formats. While I'm moving from MSSQL to MySQL, I don't need to make any "live" changes, so this is not a production-level migration issue so much as a redesign.

For instance, dates are stored the same as tags/subjects about the articles, but in strings as "YYYYMMDDd" (the lowercase d stands for "date" in the string). Essentially, before or after I move from the previous database format to a new one, I'm going to need to do a lot of replacement of individual entries. While I understand how to do operations with regular expressions in non-database issues, my database experience isn't robust enough to know the best way to handle this.

What is the best (or standard) way to handle major changes like this? Is there an SQL operation I should be looking into? Please let me know if the problem isn't clear--I'm not entirely sure what kind of answer I'm looking for.