The good news: when the change happens, Sebastian is in the forest. This means he doesn't have time to be as angry as he'd like about it.
The bad news: he's quite deep into the forest, and realises immediately that is no place for the pathetically weak, half-blind, half-deaf insect he's suddenly been reduced to. As fond as he is of his arsenal of cutlery, he knows there's a reason humans seldom use butterknives and forks as weapons.
So, yeah. Booking it out of there it is. Feel free to run into him in the forest? Alternatively, he's to be found at the outskirts of it as the barrier breaks and the monsters within pour out - pathetically human or not, he's at least managed to kill the small monster that stalked him out of the forest (charmingly, via a knife to its eye). But that's one small monster, and there are more, bigger ones coming.]
[b; 'look at this disaster' option
As it turns out, humans aren't made to work around the clock with no food or breaks to rest. Who would have thought.
Sebastian, however is... well, Sebastian. It's three and a half day of working non-stop at the farm to harvest as much food as possible in these unstable conditions, and of stacking canned food in the underground cafeteria for the entire population (needless to say, he's rationed it and everyone who pays attention should know about it), before you can find him down in the aforementioned cafeteria, leaning against a counter with his chin in his hand, half-nodding off.]