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.