Oops!

No need to drain the oil - put the bike on a slight gradient or let the side stand sink a little into some soft ground (be careful it doesn't sink too far obviously! Maybe get an assistant.)

I've had the whole side of the crank case off like this without draining the oil and got very minimal oil leakage over the space of an hour, just caught it in a drip tray. You shouldn't have a problem for 30 seconds swapping the filter cover!