OK guys, the snow is clearing from the garage roof and I'm ready to put the 300D back in it and get back to work. It's drooling coolant from the t-stat housing, enough to make a mess I'd rather not have on the garage floor, so I want to change the gasket. My original plan was to drain all the coolant and replace it, but given the weather and a concurrent storm of other problems, I'd rather save that for a month or so and just do the t-stat gasket. Any reason not to do it that way? I.e., is it necessary or recommended to drain the block before doing the t-stat? Or can I just put on goggles, remove the housing, replace the gasket, put the t-stat back in and start it up?
The coolant does need to be changed -- it's about four years old -- but is and has always been the correct coolant.

__________________
1987 300D, arctic white/palomino--314,000 miles
1978 240D 4-speed, Euro Delivery, light ivory/bamboo--370,000 miles
2005 Jeep Liberty CRD Limited, light khaki/slate--140,000 miles
2018 Chevy Cruze diesel, 6-speed manual, satin steel metallic/kalahari--19,000 miles
1982 Peugeot 505 diesel, 4-speed manual, blue/blue, 130,000 miles
1995 S320, black/parchment--34,000 miles (Dad's car)