Flatten network to aid data center move

Has anyone had experience with trying to "flatten" their network (e.g. extend subnets across geographics).? We would rather not re-ip every server before moving to a new location because of the likely unexpected results. However, we are also concerned about the impact to the network of such a change and concerns over spanning tree convergence. We also know that considerable work will have to be done after the move and that we will miss a chance to build a new, better architecture.