Pls see the following document for the detailed version to version migration:
UmbracoDocs/11/umbraco-forms/installation/version-specific.md at umbraco-eol-versions · umbraco/UmbracoDocs
We migrated from v8 to v15 (with v10 and v13 intermediary steps. In our case, it was not possible to upgrade with nuget - it stucked on some v8.x and for some reason could not be upgraded to v8.y. Finally I have removed all the forms tables from the database and installed the plugin from scratch (in version 8). Then imported all the forms definitions - we kept them in files originally and followed the migration as described in the above doc.
In our case, it appeared that PreValueTextFiles haven’t been imported to the database in the process, and required adding the file in /umbraco/Data/umbracoForms/. Fortunately it was the same file format, so just plain file copy between the original and upgraded instance worked well.
Oh, btw - please be aware the default form templates have been removed from the forms plugin and now you need to manually re-create them or download default ones. You can find the list of links for all forms versions here: Themes | Umbraco Forms