My P6 is linked from P7 at the moment but once P7 is "fixed" with SR1 I will be moving my P6 stuff over. I've always used one runtime, I'm not a big fan of multiple runtimes and I have tried both!
That said I wouldn't recommend a "pick up and dump" approach, you will need to take some care in how you do it but it can be done, I have done this since version 5 and I have migrated some stuff already to make sure it can be done but the rest will be done later!

Injustice will be avenged.
Cofiwch Dryweryn.