The “Migrate Settings” tool in Inventor 2020 does a really good job of pulling through most of the settings that you need from Inventor 2019 (or earlier), but it does not bring over any custom iLogic Snippets that you have created.
The good news, is that this is really easy to do manually.
From any iLogic rule in Inventor 2019, click on the “Custom” tab in “Snippets” and then choose “Save custom snippets to a new file“.

From any iLogic rule in Inventor 2020, click on the “Custom” tab in “Snippets” and then choose “Merge another snippets file into current set of snippets“.

This will merge any changes from the default snippets .XML file, and effectively import your customisations.

The final, and most important step, is to save these new settings. You will need to save the new XML file somewhere safe, I have placed mine in C:\Autodesk.

The animated GIF below shows how to do the migration, if you click on the GIF and HD version will load.