Is there a way to know when ota updates for our switches exists? Is an email sent out? Is there a way to see the change log when they are released? Are there instructions anywhere on setting up ota with a zwave stick when using smartthings?
Right now, the answer is easy: there aren’t any. 
I’m not sure how they plan to address this in the future. I read once that they were planning on releasing two updates a year (!), which seems a bit aggressive and perhaps overly optimistic, but we’ll see. I’m positive they will provide a changelog.
For SmartThings, it’s quite likely you’ll be able to get OTA updates via the hub — SmartThings is testing this feature for rollout at some point (doesn’t exist for Z-Wave yet, already does for Zigbee), and Inovelli hasn’t said but has hinted that they are working with a large manufacturer on testing OTA Z-Wave updates, so my guess is that this would be the one. If it’s like other ST updates, you really won’t get to choose when these happen, just if OTA updates are enabled, in which case you’ll just get it whenever ST decides they want to.
For other platforms (or even ST if you prefer), they’ve hinted that a Z-Stick or similar Z-Wave USB device coupled with PC software will work. This is how many other manufacturers allow updates. One way to do this would be to exclude the device from ST, pair it to the stick (may require software on the computer to put it into inclusion mode), do the update (will definitely require the software), then exclude the device and re-pair it to ST. You’ll have to set it back up in all your automations since it will appear as a new device; you might be able to get away with not excluding it, doing a reset, then using ST’s “Replace” feature for Z-Wave when re-pairing it (ST doesn’t exactly brag about this feature, but it’s required certified Z-Wave controllers, which they are). You could probably also add the stick as a secondary controller and skip some of this, but that’s not quite as easy from the Z-Wave side. Other hubs/controllers might already use an external stick (most Home Assistant users probably are, many HomeSeer users might be, and an external stick is optional with Hubitat and is how the hub originally came, to name a few).
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