If you want to lock in the wattage and not risk it getting knocked around, set one of the memory slots to the wattage you want, then just use that as the up/down buttons will not longer be able to change that on you. TBH I've not had that happen to me yet, even bouncing around in my pocket, but I can understand the potential concern.
As for the old/new prompt, I just always hit new even if it's not. The only time that could be an issue would be a tank you're using in TC mode, and you remount it while it's still warm from use on the mod.
As for the old/new prompt, I just always hit new even if it's not. The only time that could be an issue would be a tank you're using in TC mode, and you remount it while it's still warm from use on the mod.