How to bulk convert ISO8859 text in Text columns to UTF8 in nvarchar(max) columns?
SQL Server 2008r2 dopes not support UTF8 (such support does not arrive until the upcoming 2019 release) it only supports UCS2 (essentially a subset of UTF16). Because of this there is unlikely to be a practical solution in SQL Server itself.

You may have to code up a tool to pull the data out, convert it, and update the DB with the newly converted values.

On rereading the question, I notice you mention mysql as the target database, though your tags do not mention that. The types you mention (NVARCHAR etc.) are not mysql types. You may need to clarify the question and update the tags.