Upgrading from Sitefinity 7.2 to 9.1
Sitefinity is a very nice & friendly CMS but when it come to upgrading it from an older version, to a new version always ends up as a roller coaster ride. Unexpected things tend to happen as expected.
Following are the two main reasons, your one-click upgrade might fail when you try from 7.x to 9.x versions.
1) Dynamic Module permissions - New feature related issue
A feature introduced in 7.3 - requires the upgrade process to do necessary changes to the database and as well as populate the default permissions for dynamic module items. So if you have large sized Dynamic Module or Modules then it will take a very lengthier time and the upgrade may result in timeout error (Error message in the Sitefinity Log file will be damn scary).
This is expected, and the solution is as below.
Even if the upgrade times out, you can re-continue the failed operation by using the code snippet mentioned in above Knowledge article.
2) Type not found error
Type not found in the system TypeRegistry. Register it there first. Type: Telerik.Sitefinity.Pages.Model.ControlPresentation
Sitefinity has many modules and those modules may fail to register their types due to an interrupted upgrade or some interference.
So in those cases, when you see a module name in the error. Simply go to the System config > remove the version number specified for that targeted module. Above message shows that the Control Templates module has failed to register its types. So remove the verion number from the excel and simply - restart the server.
Sitefinity will verify & do necessary registration and updates as required.
Finally to avoid unwanted hassle, first see through the list of gotcha's listed in their upgrade page and then start with your upgrade. That will make your life easy, because we will know for sure - that this is the way the upgrade process is build to fail and then corrected based on user intervention.
I hope that this upgrade page will stays shorter and doesn't grow with the time ...