I used the same private address range but didn't use any of the same addresses.  When I ping from a workstation in the "new" switch to a device on the old it is hit or miss, sometimes none are dropped and sometimes all are, and sometime some are dropped.  When a ping does answer the first one always has a high time.

Hi Eric

Do you have any network diagram or base architecture , explaining your setup ? do you have the two datacenters connected over a layer 2 trunk ? are you moving your devices from old datacenter to new datacenter physically ?

Raj

Just one datacenter, switches are right beside eash other.  They are connected to seperate routers, I just attached to two switches together with a cross-over cable.

I might not have a clear picture of this in my head, but per your diagram, wouldnt your routers have issues with

having the same configs on them, so when you connect the two networks together, how does your network know what path to chose?  Or do you have just one router onsite?  From the diagram, looks like you have two when you connect that cable.

Hi Eric

Have you configured the interconnect between switches, as Layer 2 ? what is the default gateways of the PCs behind the old network and the new network ? one simple way is to configure HSRP between the two switches and configure the default route for the PCs as the virtual IPs between the old and new datacenters.. by doing this your migration from switch 1 to switch 2 will be easy and transparent.. but are these layer 3 switcehs ? if you have drops on the interconnect link, can you confirm if the link doesnt have any errors or CRC on it ? can you post show interface and show ip route from the new switch ?

Raj