Serial Tubeless Tyre Swappers...

I just pop the bead off at the bottom of the tyre (holding the wheel off the ground) and use a syringe to suck the sealant out & squirt it back in the bottle.

I have an old 60ml bottle of Schwalbe sealant which has a nozzle on the end; you can use it to suck up all the sealant.

I use the old red cup that used to come with Stans to scoop it, straight into the new tyre. Gets most of it out. 

Those little plastic cups you'd buy in a tube of 25 or 50 for squash at kids' parties.  Nice and flexible and conform to the tyre. Gets most of it in one scrape.  A syringe would help grab the last but normally I don't bother.  

They're cheap and we use them for model making and other stuff.  I know it's plastic but we do use them until they fall to bits.  

Valve core out - syringe it in and syringe it all back out with the tyre still seated, zero mess, zero faff

60ml Stan’s bottle like the Schwalbe one ☝️ 

syringe would be the best way, but I already have the bottle 

If it's been in there for more than three to four months I just tip it into a bottle and chuck it

Scoop out with aerosol lid into new tyre 

Try to pour it from one tyre into the other making a big mess and getting 50% max into the new tyre….I'm going to adopt one of the techniques above, thanks!