Hi Daniel,
You have actually asked a very wide question with no information related to the funning configuration on your existing boxes and the throughput requried by those application. which modules of BIGIP are you running on 6800 units ????? v 9.2 any ways is really very very old... and switching to Vipron 2400 is not a bad idea... your F5 Local SE should be able to guide you properly and make sure that the application that you will be switching from the previous version is properly working on the new version. I am sure you are aware that it is not possible to take the configuration file backup on ver 9.2 and just upload it on ver 11 its not gona happen so better get in touch with your local F5 S.E.
Regards,