It would not be possible to switch the server onto the VDS if you converted your package, as it currently stands. The current server would be deleted as part of this process.
If you adjusted into a VDS, you would only pay the difference in price between the two, prorated to cover the remainder of your time.
The performance of a BF3 server on a VDS would be at most equivalent to the performance of a standalone game server. We don't officially support servers over 48 slots on our VDSes because of the CPU resources that it uses and the variance in VDS-hosting machines: viewtopic.php?f=70&t=5815