Next steps expanding Drobo capacity

Right now, I have a 750 and 3 x 1Tb drives in my 2nd gen Drobo. I was looking last night at the next step in adding capacity. I'm looking at replacing the 750 with a 2Tb drive next. This doesn't expand the drive though. I'll need to replace one of the 1Tb drives also to do this. This will probably be a weekend affair. Pulling the 750 and inserting the 2Tb will likely mean 2 or 3 days or relayout. Repeating that for the 1Tb -> 2Tb is the same so I'm not looking forward to that.