When you install the latest version of Joplin, you will be asked to upgrade the sync target. This essentially means uploading a new setting file to your sync target, whether it is Dropbox, OneDrive, Joplin Cloud, etc.
Once this is done, only apps v2.3+ will be able to sync with this upgraded sync target. So before doing this make sure that you are ready to upgrade all your apps to 2.3.
I use Dropbox to sync between mobile and desktop. I'm not sure what "update the sync target" means for me.
Does it mean if I upgrade my desktop Joplin app (which I planned to do today) then I also have to upgrade my mobile app (Which I haven't done for 6 months and would rather not have to)?
If you upgraded to the desktop pre-release (2.3.1+ atm) then yes, you'd also need to upgrade your mobile app (also to pre-release) or it wouldn't be able to sync.
The current desktop release, 2.2.7, no, you wouldn't need to upgrade the mobile app.
Yes all clients would need to be on 2.3. But I haven't even pre-released 2.3 desktop yet because I want to use it for some time first to make sure it works before releasing it.
I wonder if there is a way to more clearly indicate in the subject line of posts that you are referring to pre-release things. Those of us who aren't hard at work like the devs aren't so conscious of all the release levels.
EDIT: or maybe "ATTENTION DEVS"?
Thanks for all your hard work!!
This post was meant to caution people not to install a desktop pre-release >= 2.3.1, unless there's also a mobile and/or cli of >= 2.3.1 available.
Isn't it standard to put something like a lower case b at the end of per-release versions? I'm actually frequently confused by this on the forum.
@whitewall this is not meant for devs, but rather for users.
Let's say pre-release 2.3.x or release 2.3.x is released. You install it. But there's no mobile app yet released. The first thing you will do is to open a github ticket or a forum post that you can't sync with your mobile client anymore. It won't stay at one post, but there will be at least 5 posts, because people don't search for anything before posting.
This is a warning for pre-release and release. Upgrading the sync target has to be acknowledged in the app and the version is also mentioned in my OP. I am not sure how much clearer the warning can be. However, I'm open to suggestions.
I think your post could have more details for non-developers. For example instead of saying "DO NOT" do this, explain what should be done. For example, making sure you're ready to update all your clients before installing 2.3.
Of course it's all the more confusing because there's no desktop version 2.3, not even a pre-release (I was going to announce that in the release notes).
I am really not sure how. When people install >= 2.3.1 (whenever it will be available) they are asked to upgrade the sync target.
My post says that they should not do it, unless all the clients are at least on version 2.3.1
So, if they are not asked to upgrade the sync target, all is good. If all their clients are >= 2.3.1 and they are asked to upgrade, all is good.
Maybe I am missing something here, but the sentence "Do NOT upgrade the sync target unless all your clients (desktop, mobile, cli) are at least on version 2.3.1." says all that.
Maybe I should not have posted it, but I thought it would be better now than too late.
Thanks for the heads up! I use CLI as well as Desktop and Mobile, so it's useful for me to know. I misunderstood your initial post, as to me, sync target means Joplin Server, or switching sync targets in the settings (from Dropbox to e.g. Joplin Server). It sounds more like the syncing method, regardless of target, is changing so it'll be incompatible with older client versions. Did I get that right? 
Yes I mean for me your post makes sense but could be clearer. It's still better than nothing as at least it brings some awareness to the future issue.
The message you get is: The sync target needs to be upgraded before Joplin can sync. The operation may take a few minutes to complete and the app needs to be restarted. To proceed please click on the link or Sync target must be upgraded! Run %s to proceed.
This is why I called it sync target in my post.
P.S.: I clicked reply on the wrong comment. This was meant to be the reply to ATTENTION: sync version has been changed in 2.3.1 - #12 by wazabees
ok, I will try to clarify it.
@whitewall @wazabees @laurent does the additional info clarify the post?
I think the bit that would confuse me is whether the sync upgrade is actually optional.
If I open Joplin, do I actually have the choice to not upgrade the sync target. What happens if I don't?
If there's no choice, I think it's better to rephrase it as literally just don't bother upgrading Joplin, rather than upgrading the sync target, because you can't cleanly seperate the two and not upgrading Joplin entirely is a more direct and clear instruction.
The sync operation won't complete and no syncing will be possible until you do.
"you will be asked" implies a choice or is my English completely off?
No, your English is fine :P. It certainly implies a choice, but the issue is you can interpret that choice in the same way Windows 10 will ask you to update, and it will ask, but eventually it's going to ignore you if you keep saying no.
I think the other question I'd have as a user, if I upgrade one client to 2.3+ and another remains on 2.2, if the sync-target is still version 2, do they sync then?
Writing is hard, so personally I think the current post is fine but there'll always be people who get confused, unfortunately, writing too much is often just as bad as writing too little so I'm not sure it's something that has a perfect solution anyway.
Edit: Re-read it and the last paragraph implies the answer is yes, they'd sync on version 2.
I made small tweaks, I hope that's ok. "Client" => "App". "2.3.1" => "2.3" (because there won't be a 2.3.1 because it was broken). Also tweaked the last sentence a bit.