Now I'm using UM2.3.2 and it'll be upgraded v3.1 next week. As for billing, How can UM bill and gather the vRAM information during upgrade ?
The collections are not 100% constant ... I think they are once and hour. You can track this from the monitor page. Really, you should power off the 2.3.2 appliance, take a snapshot, attempt the upgrade.
If everything works, then you commit the snapshot.
In addition, during upgrading, Can I deploy two UM instances both v2.3.2 and v3.1 on the same vCenter ?
This is also fine. The amount of load the UM puts on vCenter is negligible. However, there is no way to migrate data from one usage meter instance to another. Also, remember that you have to keep the data around for some time. this means if you did deploy 3.1 side by side to 2.3.2, then you have to run them in parallel until the next billing date. this will give you time to ensure that 3.1 is setup correctly and collecting data.
once you know that it works fine, just power off and backup the 2.3.2 appliance for archive purposes to get data from it if you ever needed it.
