You can fill up with 93 now, or once you installed your tune, you'll be fine, just once it's installed try to only fill with 93 to prevent any drive-ability issues that could arise.
You can disconnect the positive cable, As long as you disconnect power in some way. Once it's disconnected I'd wait like 10 minutes or so before actually unplugging the PCM, just as a safety measure. I've swapped by ECU's out a couple times. I've disconnected the battery every time, someone told me once that the computer stores power for a few minutes in case the air bags would need to be deployed. Not sure if that is the case or not, but never hurts to just wait a few minutes and let it discharge.
Aside from the 10mm to disconnect the battery you won't need any other tools. Simply a measure of pulling back the plastic clip to remove the PCM, and just unplug and swap in the new one!