The solutions for next step might be works, ex 0.8.1, might be:
First limit the block size generate by miners, with the same size of 0.7's DB, other are the same with 0.8.
After all client are upgrade to 0.8.1, then upgrade to 0.8.2 with the feature:
remove the limitation of the block size 0.8.1's miner can generate, and the 0.8.1's DB can accept the larger block size, require that there is almost no 0.7 client now.
My opinion about the limitation is:
The block size miners generating should separate from which clients accepts(sync).
So, The upgrade for miners should also separate from common clients.