I think you can just leave the 32-bit version alone, let it live on the blacklist. When you install the 64-bit version, Cubase should pick it up and you can use it. You may have to rescan the plugin list using plugin manager (or by restarting Cubase). In Windows, the 32-bit plugins go in a different directory from the 64-bit versions, and the plugin installers normally adhere to this convention; "Program Files (x86)" is in the path for 32-bit, "Program Files" is in the path for 64-bit.