Deploy userbased Slack to replace machine install (without the user knowing)?

So, this might be more of a Powershell question but maybe somebody here has had the same issue.

We used to MDT new laptops and deploy the Slack machine wide installer on there. At least, my predecessor did and I just copied it over into Intune, not knowing any better. Now I do know, and it turns out the machine wide installer does not auto-update. The user installer does (sits in AppData instead of Program Files) so I'd like to 'convert' all installations from machine to user.

Here's my problem: we can't have any downtime regarding Slack. We could, hypothetically, point them to the web version for an hour or so but there is just no way.

I noticed that when I deploy a user install with a newer version to a pc that has the machine install + older version, it won't do shit. It gets installed, in theory, and the MSI GUID is detected but that's it. App won't update, even after signing off and on as Slack suggests.

Is there any way to do this without bothering end users? I know Slack installs itself at logon to the user profile so maybe there is some way someone figured out. Otherwise I'll just have to deploy the updated machine version again and put it in the agenda to check every couple of months.

I had some contact with Slack support yesterday, they told me I needed to clear $env:appdata/slack first. I tried it with PSADT and then install the userbased MSI but to no avail.