Nothing that affects compatibility has changed. The only pitfall is that new tables that are created with a DATE type will use the new way to store the date. This means that you can't access this field from an old mysqld version.
The C interface to mysql_real_connect() has changed. If you have an old client program that calls this, you must place a 0 for the new DB argument (or recode the client to send the db element for faster connections).
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