The user control properties that are not migrated must be properties of widgets that are not of the widgets that are migrated by sitefinity from 3.7 to 6.x versions (widgets that are available in both versions) or the properties that are not migrating are not properties available for the same widgets in sitefinity 6.x version.
Please let me know what properties for which widgets are not migrated? If the widgets which properties are not migrated are custom versions of the built in widgets for sitefinity 3.7 (like customized widget for displaying news) which was added additional properties to serve specific case those additional properties are not to be detected by the migration as properties for news widget, in this train of tough this will be the same for other customized widgets.
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