A data migration throttle that supports the incremental and reversible migration of web site customization data from a client-side data store to a server-side data store. Initially, each user of the web site stores customization data, such as the user's zip code, in a cookies file together with a globally unique identifier (GUID) which identifies the user. In order to migrate data to a server side store, the throttle is set to a value. Each time a user contacts the web server a hash of the user's GUID is compared with the throttle value, and the user's data is migrated to the server if the hash value is less than the throttle value. The throttle may be lowered, thereby causing some users to revert to client-side storage of customization data.