I have pseudo-ownership of a fairly old db (original data from 30 years ago; current design is >15 years old). In my opinion, the schema is pretty broken, and one of the implications of this is that there are a lot of inconsistencies/issues with the data. I'm planning to write a new schema and port the data across, which is a fairly simple task as new data comes in rarely.
Would you attempt to fix the inconsistencies in the old database first, or iron them out as part of the migration process? I'm tempted to go with the latter - since I'll need proper validation anyway, and some errors will pop out naturally with the different schema design - but fixing the data first would break the task into smaller chunks and allow people familiar with the old db to validate the fixes.