In code, it's generally pretty easy to add new classes to provide additional functionality and such. I have a fairly good understanding of refactoring code and what's involved so YAGNI generally makes sense to me.
What I'm not as familiar with is working with and updating a relational database once it's deployed. I'm developing a little pet project that I'm planning on practicing Release Early, Release Often on and I'm wondering if I should be considering data that won't be used in the initial release, but is on the planned features list? Is it as easy to add tables and tweak schemas around as it is to add new classes? Or should I try to have tables set up for things I could conceivably use, but aren't planning to in the immediate future?
If you have good testing that hits the database, I would extend YAGNI to your database design.
In general, it is easy to add columns and tables, and less easy to remove or modify them significantly. Take that into consideration when you design tables (i.e. if a customer can have multiple users, don't add userid to your customers table. Do it right the first time).