This thing is the bomb. Remove old sealant, and inject new sealant with an absolute minimum of mess. A steady hand may be able to get it done with zero drips (The current record in the Pure Gravel office is 2 drips).
From the manufacturer:
Other tire sealant injectors try to make a seal over the valve, creating a big ol’ gunky mess. Our tubeless tire sealant injector goes directly through the valve and straight to the tire. No mess, ever.