Hello John,
I've been through the same excercise. What is more important is how much free space you have on your VaultStore disks. Upgrading to 64-bit indexes means a re-index of archived items. This means indexing has to go throught each and every item, meaning an extraction of cab-files (if you use these), and a load on the server.
How I approached it:
When upgraded to EV10, rightclick your EV-servers in the console, properties, tab advanced, set value "Maximum concurrent indexing capacity" to 10. (Default = 30). If you don't, and start upgrading indexes, you choke the server (at least, I did, and my servers are 16 to 32 core, with 32GB ram at least).
Upgrade indexes controlled. I initially started with 'indexes starting with A...', but then decided to start with the smallest. In the Upgrade Wizard, you can sort on several columns. Sorting on items will give you that choice. YOu can also do per Vault Store etc. YOu need to play with this to figure out your best approach.
I did 1200 archives with less than 1000 items in 90 minutes. The longest upgrade was a journal archive having litlle more than 55.000.000 items in it, that took 12 days. (of which about 10 hours were used to run, rest was backupmode etc). I choose to do user-archives first, then the Journal ones.
What happens during the upgrade?
If a 32 index is selected for upgrading, it is closed. A new 64 bit index is opened. The 64 index is being created. As soon as it is finished, EV checks to see if it is ok. If ok, the 32-bit index is being deleted.
You will see that after your upgrade, all archives will start getting 2 index-locations. EV starts using the 64 bit indexing for newly archived items. Users will not notice...
I found that the index disks grew about a mere 3% (we already had full indexing), which was easily possible. Also remember that you can close a 64 index location, and that that is really closed. (no new data added). I had little to no failures, from the approximately 22000 archives I upgraded, only 3 really failed (due to corrupted store location, or missing files). As these were archives for 'leavers', I left them as is.
Hope this helps.