The upgrade includes a new version of Data Assist and a new version of Security Assist.
Had Data Assist 3 been released as a stand alone extension, the upgrade price from V2 to V3 would have been $149.99. The Upgrade price for the stand alone version of Security Assist From V1 to V2 would have been $75.99.
You save $25 on the upgrade to Data Bridge.
In Data Assist 3, there is tighter integration from within the wizard with the other tools like HTML Editor and File Upload. Wile configuring the Data Assist Wizard, you can Specify fields to be File Fields, and the upload behavior is automatically added to the insert and update pages. HTNL Editor can also be added directly in the wizard. those are just a few of the upgrades in Data Bridge.
if you don't wish to upgrade, you can continue using the previous extensions in the previous version of Dreamweaver.