Which version of Moodle are you using? Most / all such problems should be fixed by Moodle 2.3.2. As someone else said, if you find some that aren't fixed in the current version, it would be best to create a tracker issue with your proposed fix and let other people look at that specific case.
If you're fixing an old version, have a look at how it has been fixed in newer versions. Look at the diff in the tracker issue.
It needs a close look because changing from ntext to nvarchar will make the column comparable against other nvarchar columns, but make it not comparable against the columns it used to be comparable to! So you could fix one problem, but create three more in the process. Submitting a tracker issue and letting other people look at it will reduce those problems, and it'll get the fix into the next version so that it continues to work when you upgrade.