When you need to switch a fields collation you can check if any existing data will conflict with this change.
For example a keyword-field might contain data that is considered unique in one collation but not in another. The Swedish character ä is considered a unique character in swedish collation but in general collation it is only an a in an accented form.
Comparing the results from these two queries will show what effect, if any, an change in collation will have.
SELECT id, keyword AS KEY, count(*) AS num FROM keywords GROUP BY KEY HAVING num > 1; SELECT id, keyword COLLATE utf8_unicode_ci AS KEY, count(*) AS num FROM keywords GROUP BY KEY HAVING num > 1;