You still have MySQL Servers in MySQL Cluster and you can mix and match storage engines (e.g. use InnoDB for some, Cluster (ndb) for others). Cluster 7.2 uses a tweaked version of MySQL 5.5 for the MySQL servers and so if you're already on 5.5 then the migration is fairly simple...

0) Always a good idea to backup! 1) Create your cluster... http://www.clusterdb.com/mysql-cluster/deploying-mysql-clust... - stop before starting your mysqlds 2)Configure your mysqlds to use the same data directory as your existing ones 3) Stop your existing mysqlds 4) Start up your new (cluster) mysqlds. At this point you still have your existing tables stored in the original engine (e.g. MyISAM or InnoDB). 5) Migrate appropriate tables to cluster... "ALTER TABLE <tab-name> ENGINE=ndbcluster;"

Schedule downtime, dump DBs, backup EVERYTHING three times, import DBs into new RDBMS, switch over config and bring everything back up.

You know who has scheduled maintenance? Apple, Verizon, Sprint, etc.