Metalogix Content Matrix - File Share Edition has a special dialog that allows you to edit Long String column type values into more "readable" text prior to migration..

NOTE:  A "Long String" column may be displayed as Read Only, however, these values can still be edited.

To edit Long String column values:

1.In Explorer View, select the folder containing the items whose long string column values you want to edit.

2.Switch to Items View.

3.Select the long string value that you want to edit

4.To display the Edit Text - Long String dialog, either:

§Double-click  to enable editing.

OR.

§Click the Show Properties icon then click the [...] button that displays to the right of the selected property.

5.In the Text Visualizer tab lets users edit the contents of the long string property.

NOTE:  If the long string text value is in structure XML, the XHTML Visualizer tab becomes available, which is useful for viewing the value as structured XML, as well as rendered content.

6.To view/edit a different long string column properties for the same item, select it from the Property drop-down.

Note that only long-string properties are listed in this menu.