To 9.0 yes. To go to 9.1RC2 go to 9.0 first (as mentioned on the 9.1RC2 cautions page). If you go to 9.1RC2 you will also need to change a setting in the vloader:
setenv console vidconsole,comconsole
The cluster upgrade method will not work because that method expects HA pairs, but the traditional rolling upgrade method will work with some care given to epsilon and juggling of svm root volumes. Also ensure you have sufficient space on the node root volumes to download the image.
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