As Saroj points out, the upgrade/migration procedure is the best path.
One thing to keep in mind is that there are certain options that are set when migrating to keep IQ 16 in a 15 compatibility mode. All that is documented, but you should be aware of it. If you wish to take advantage of better compression via the new n-bit indexes, you must set those options back to default for IQ 16 and rebuild the indexes.
Don't forget to read the IQ 16 Sizing guide here: http://scn.sap.com/docs/DOC-41455
There are a lot of changes to IQ 16 with respect to memory. If you treat IQ 16 as you did IQ 15 you may not realize all performance enhancements and gains in the new version.