Well Kate, I can't tell you what the best way to go about it is, but seeing as how we just went through the exact same scenario, I'll tell you how it played out for us.
I had no intentions of moving T from his crib for as long as possible! We actually got a second crib from my cousin, so there was no pressure to get him out of his.
F was born end of April, T was 20mos. We moved the first time a week after F's birth. For two months F slept in the bassinette next to me and T slept in his pack n' play, so still 'crib-like'. We moved the second time in July and put T back in his crib, intending on him staying there for awhile still. Within weeks his sleep went to crap so we said to heck with it, took his mattress out and trialled him sleeping out of his crib on the floor. That went ok, so initially we converted his crib to a toddler bed, but it was pointless really. In September just after he turned 2, we got him a BBB, just a normal twin bed with a bed rail. He loves it, sleeps like a champ!
So yeah, it all worked out even though we had no plan and no idea what we were doing. Is Austin the type who will get out of bed and wreak havoc, or is he pretty easy going? I might be tempted, since there are so many changes anyway, to just put him in his BBB right when you move. Things will be out of whack anyway. But then again you will have a baby to contend with. I don't know, I just know it worked out for us because T never gets out of his bed! lol