Moving users to upgraded Moodle

I'm upgrading our systems - using Joomla 3.8, upgrading Moodle from 1.9 to 3.5 & Joomdle from 0.95 to 1.2. Is it possible to move Moodle 1.9 users through Joomdle to Moodle 3.5 so all emails/usernames carry over? I'm aware that passwords won't carry over. Should I upgrade Moodle or Joomdle first? Thanks!

Hi.

Are you really upgrading, or you are about to install Moodle 3.5 and then migrate data?
If you are upgrading, you would not need to migrate any users, as all of them would already be there.

If you are upgrading Moodle, best thing is to remove Joomdle first, and then install new version once Moodle upgrade is complete.

We're installing Moodle 3.5 and migrating data. Can we unlink Joomdle from 1.9 and link to 3.5 once it's up and have users migrate?

Yes, you should be able to do it. although it would not be Joomdle doing user migration.
I think it is better to use some user migration tool, or just dump data directly, although you will need some knowledge for this.
Once users are in new Moodle site, you can install and configure Joomdle, and users will be already connected.

A link I found that can help:
moodle.org/mod/forum/discuss.php?d=345378

There the problem is passwords, but password is not a problem for your Joomdle users, as Moodle password is not ever checked for those users, so you don't mind if it is not correct at migration.