How to manage Conformed dimensions

Hi,
If I'm implementing data mart by data mart, I build a dimension for the first data mart. After a few months, I realize I can use that dimension for the second data mart as a conformed dimension. But it needs additional columns. How do you manage this? Do you just add the new columns in the table providing these new columns for the first data mart as well?
How would you handle it if the granularity is different?