hi,

here is what i would recommend. This is just an outline. Obviously, if you can, test this out in a non-prod environment first.

• download the software
• you might need a different license key since it is a major version change
• there is a installer script, run the pre-check option, it should tell if your patch level are correct and if there is enough space to run it
• haconf -makerw, you should freeze the systems with hasys -freeze $node -persistent on all nodes
• also freeze all servicegroups persistent, save your config with haconf -dump -makero
• verify your main.cf to see if the FROZEN lines are indeed there

• then use the installer to upgrade the product. 
• patches for 6.0.1. i think there isn't any yet for vcs but there are some for vxvm and vxfs. 

• once the install is successful and vcs is up and running. you can unfreeze both systems and service groups

the backout plan, minimum disruption would be just reinstall the 5.0MP3

if you can have downtime, break the mirror and then restore from the mirror.

VCS changes to use AMF, with is not the agent checking every min to see if the resource is still running way. AMF hooked up to the kernel notification. I don't know if it is better. but certainly the reporting would be faster. if you don't want AMF, you have to change your main.cf I believe to not AMF.