HFM Classic Adding a Custom Dimension

We're on HFM Classic, 11.1.2.3 and we'd like to add a custom dimension to the original 4.  I found the steps to do that in the admin guide but without spending all the time first we're concerned about how it handles all the years of historical data that won't have Custom5 associated with it.  Can someone tell me what happens after you create a new custom dimension for all the years of history we have in our application for that dimension?  I'm hoping it just all defaults to [None] or something like that.

Thanks.