Auto-generate slugs for all old posts?

I changed my permalinks recently, from the numerical format (?=123) to the nice format (this-is-the-title). Problem is, my oldest posts that had been blogged in Greymatter and migrated over to WordPress do not have slugs. Instead of going to the page https://sparklette.net/archives/this-is-the-title/ it goes to https://sparklette.net/archives// instead. How do I fix this? Thanks!