I've also been wanting to switch from 20s to 30s. As far as I can tell, all I need is 2 new 30s, a longer piece of threaded rod (I think 1/2" diam, but I haven't measured yet) and a new propane tank cover. The one I have now doesn't have much space at the top so I assume if I tried to put it over the 30s it'll look like my DS's pant legs when he has a growth spurt.
If I do get around to doing the swap, I'd probably just use the too short cover to start, until I can get to the dealer and change it for a new one. If I remember correctly the height difference is around 1/2' so with the 1" or so slack I have now, plus the 1" of tray the tanks sit in, the short cover should only expose maybe 3-4" of tank at the bottom.
Of course, this is would be in my case. It will all depend on your particular cover and rig.
Edit: I just had an extra thought. You said you're picking it up from a dealer over 2 hours away? Why not just call the dealer and see what they'll charge you to change it over before you pick it up? Might be able to get away with just the cost difference of 20-30lbs tanks. That way it's all said and done before you even get there.