Breaking Changes
The What's New notes of versions 7.0 to 11.0 list no member of TX Spell .NET for Windows Forms that was changed or removed, and none is marked obsolete. An upgrade therefore changes your project, not your code, plus a few behaviors worth testing. When your application uses TX Text Control as well, read its breaking changes too, as Update Together With TX Text Control describes.
By the Version You Come From
An upgrade from 10.0 needs a project that targets .NET 8 or higher; an upgrade from a version earlier than 7.0 needs the .NET Framework 4.0 or higher. Beyond the target framework, test the suggestions (changed in 10.0), user dictionaries with flags and the scaling of the dialog boxes (both changed in 8.0). Breaking Changes in What's New describes each change in full, with a table of what to change and what to test for the version you come from, and Deprecated APIs confirms that nothing is marked for removal.