There are a few that I can think of:
1. Make sure that all your cards and services are supported. Some won't work for VSLs (6704) and some will not work at all in a VSS or with the SXI software. We found it easier to upgrade the switch to SXI2 first to make sure that all cards will work with the IOS.
2. When you perform the upgrade, your entire config will be erased and you will need to start from stratch. It will speed things up to have a config created in advance on a flash drive