All Blackberry phones are required to have a BB data plan: unlimited email, and web browser. All carriers require the "data plan" with Blackberry phones. Multimedia is part of a different bundle, with voice plan (in my case.)
The data plan you have on the Voyager is likely the now obsolete Connect or Premium plan, and it is NOT the same as the one required for a BB.
If you do upgrade to a BB, your plan will change to the current Talk or Talk&Text, and the $29.99 BB data plan will be added to that.