Basically the only difference (as Bret mentioned) would be that you'd get "stuck" with 2-bytes instead of 4, which would limit the character set. That being said, if you do not intend to use them, it should not pose that much of a problem. The conversion, on the other hand, should not be attempted on a live DB whenever possible, because you'd risk a chance of data being entered in the wrong format just in the milisecond the table is being converted.
Installing and upgrading help
MySQL full unicode support (Moodle version 3.2.3)
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