As the other commenters said, consider DataProtector 8, where the database copes much better with larger catalogues. With a fresh install you are unlikely to have the same issues that some upgraders have had.
If you get a B620, you'll have to upgrade to DP6.21 at the very least, so you'll end up upgrading anyway.
You can migrate your media information over from the old cell to the new cell by running omnimm -export_to_mcf for each tape in your old cell that has something on it that you care about. These MCF files can be imported into the new cell manager.
Moving clients between cells is very easy: essentially all that happens is:
- on the client a registry key (for Windows) or the file /etc/opt/omni/client/cell_server gets updated
- on the old cell manager, a line is taken out of the cell_info file
- on the new cell manager, a line gets added in.
If you play games with the cell_secure file, you can have it so that the client respond to commands from two different cell managers. (And by manually hacking the cell_info file, you can trick the cell managers into both thinking that the client is theirs.)