1. You only need to upgrade once, directly to the version you want to install. There's no need for installing any intermediate versions.
2. Session logs most likely. However, audit logs can grow to be huge as well, so you may want to take a look at the table size there as well. But in any case, it's still most likely session logs you will want to archive.
3. Assuming you have not edited any of the relevant VBOs, you can just import all the VBOs in the VBO folder once you've installed the latest version. Yes, the VBOs in the VBO folder are the latest and should be stable. However, there are more great VBOs on the DX too, but the ones included with the installation should be the latest of those specific VBOs.
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Dave Morris
Cano Ai
Atlanta, GA
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Dave Morris, 3Ci at Southern Company