Note to self (not yet personally tested):
When upgrading Catalyst 6500 12.2SXI IOS from modular to non-modular in dual-supervisor configuration you cannot do any kind of switchover between the supervisors. You need to reload the whole switch.
Source: CSCsb07831 (Cisco login required)
Apparently the standby supervisor will keep reloading if you try the switchover method, they say.
Feel free to share your experiences on this.
I can attest 100% that this is true. I got stuck in a bad situation not realizing this on a VSS pair.
http://showbrain.blogspot.com/2011/08/catalyst-6509-e-vss-software-upgrade.html