Changing from utf8 to utf8mb4
Main thing to watch altering tables is the index length will increase for utf8 indexed columns which might put them over limit. Adjust URL for your mysql version as some differences apply. These index length exceeded errors will show up immediately on ALTER TABLE so this will be a final bit of the migration.

On validation, generally won't be a problem. Try this on a few tables to get confidence in the idea confidence on smaller tables:

   CREATE TABLE tbl_new LIKE tbl
   ALTER TABLE tbl_new ..  {change to utf8mb4 fields}
   INSERT INTO tbl_new SELECT * FROM tbl
   RENAME TABLE tbl_new TO tbl, tbl TO tbl_old


A SELECT ... tbl JOIN tbl_old ON tbl.pk = tbl_old.pk WHERE tbl.utf_field != tbl_old.utf_field can be used to validate values.

Application behavior can be checked, and you can RENAME back if their are problems. (taking to account updates).