i just bought a S/h WR rim for my 07 YZ, so you just swap rim out depending on what your doing. WR are 18" and you can frun proper trial tyres, the michi and dulops are sticky as, the speedway 19' dont drive like they do on a slider.
IF you want to try a speedway 19 here i have some you can have for a test run. they dont work on the moderm MX, they just fishtail even at 5 psi.
JUSt keep hitting ebay up and you will find a complete wheel for you r bike in 18" for $200, its nothing in the scheme of things. And im a tight arse and i havent looked back with the S/h 18" wheel life is easier all round.