I have dozens of articles in draft state, and that need just a little polish to publish. But while the time is tight, here goes a link to a simple article about this kind of conversion.
Simply put: you may have performance issues from using NText on newer versions of SQL (be sure to set the most appropriate storage to be used in your columns, sometimes LOB can be good). One more thing to look at when migrating or converting databases from SQL2000 and up.
Explanation about both data types:
And how to identify these columns?
TLDR (too long, didn’t read): just use NVarChar(max) instead of NText. When you get a chance, convert any data for the newer data types.