[skwirl]

Are you talking about the default styles in the Project : Styles inspector for Whole Document, Tasks, Notes, etc...? If so, try this:

In the OmniPlan document with the styles you want to use, select one style and click on the little gear icon at the bottom of the inspector, then select "Copy". Now open your older OmniPlan document and select that same style, click on the gear icon and select "Paste".

Repeat for each style until you're done.