There is a blurb in the Release Notes that refers to JVM version is reverted to Gold Version if a Fixpack is uninstalled - Technote 1980538: http://www-01.ibm.com/support/docview.wss?uid=swg21980538
This morning on a 32-bit machine I uninstalled FP6 and installed FP7 without issue - when complete JVM reported as SR16 FP30. I'll watch this evening when I upgrade my 64-bit machines...
--------later comma that same day....---------
Upgraded 5 different 64-bit machines - uninstalled FP6 & Installed FP7 - all report SR16 FP30.