Just to clarify the release history of TextMesh Pro, the first version of TextMesh Pro available via the package manager was for Unity 2018.1. As per this sticky post , the first ever release of the TMP UPM package became available about the same time Unity 2018.1 beta 3 was released. Users had to manually install this version of TextMesh Pro form the package manager in Unity 2018.1 following the procedure outline in that stick post.
Prior to that, the only versions available of TextMesh Pro were either the source code version available from the TextMesh Pro user forum (available to users who purchased the product in the past) or the free / dll only version available from the Asset Store. These releases were available for Unity 2017.4 and prior versions of Unity.
Unity 2018.2 is the first version of Unity where the TextMesh Pro package is included with Unity itself. Users still need to add the TMP Essential Resources and (optional but recommended) TMP Examples and Extras in their project but the package is included by default.
I am not sure how the 2018.1 package manager version of TMP could end up in Unity 2017.3 or 2017.4 but for certain it would not work simply due to the use of assembly definitions and some other stuff.
In terms of the upgrade procedure for previous source code version or the free / dll version of TextMesh Pro, the process has always required removing the previous version by deleting the old “TextMesh Pro” folder. This has been the procedure since the first release of TMP since 2014. This is also the same procedure when migrating from the source code / free dll version of TMP to the new package manager versions. This procedure is also outlined in the sticky post .
Although the migration from the non package manager version of TMP to this new TMP UPM version can be tricky / problematic, once running on these new package manager versions of TMP, upgrades to subsequent versions of the TMP UPM package are so much easier.
With regards to the issues you have run into, I certainly apologize for that. I still don’t know how you ended up with the 2018.1 version package manager version of TMP in Unity 2017.3 or 2017.4. This would most certainly cause all kinds of issues.
It looks like you are migrating to Unity 2018.2 where the TMP UPM package is included by default. Like I said above and as per that sticky post, you will need to add the TMP Essential Resources. If you are looking to migrate a project that was using an older version of TMP to Unity 2018.2, be sure to follow that outline procedure and review the Updated Notes at the bottom of that first post.
If you run into any issues, I am always available to provide assistance.