In order to use your old JPv2 saves, you would need to know what serials are stored in the save files, erase your DC flash memory, and then store those serials in the flash memory. After that, your new DC will read your old saves.
If you use shared serials, you won't be able to participate in server events or use automatic character backups, but you can still play. You won't be able to use your old JPv2 characters, though, as the shared serials stored in the flash memory won't match the serials in your old save files.
If you want a US serial, USv2 is cheap on eBay, but you won't be able to use your old characters unless you buy a second Dreamcast.
If you want to use your old JPv2 serials, you will be able to use your old characters online with the US CD-R, assuming your old characters have gone online before. In order to create new characters, however, the first time you go online, you will need to use your JPv2 GD-ROM and an Xploder cheat disc with the code to disable the HL check, so that you can go online.
When you try to go online, the game will ask you to enter your serials again, but because you're using the JPv2 disc, the game will accept your JPv2 serials. Connect to the server, then quit the game in the lobby, and power off your DC. Go back to using the USv2 CD-R, and you will now be able to take your new JPv2 character online with the US disc, as the game only asks you to confirm your serials the first time you go online; it won't ask again for that character.
"Fear the HUnewearl."