That is why you should make a backup of the database and files any time you upgrade or alter the database in case something bad does happen.
Anyway it alters the database only to accept the data for the emoji. as well as other 4byte items
Also as I recall there is a convert status showing what is going on but it is fairly fast and painless even though it makes a lot of alters to the database.
Without the tool it probably would take several hours to complete and then still miss some cell format somewhere..
