Hi @craneman1,
There are just too many variables to consider which is why you are not finding those kinds of resources or Apps.
Without a drawing and understanding a bit more about your environment its hard to suggest a strategy but the answers to a few questions would help.
1. Is the Data Center dark fiber (I'm assuming) providing a Layer 2 or a Layer 3 or both interlink between the two Data Centers?
2. Do you have enough rack space and environmental (power, cooling) to bring up the new kit in parallel?
If you want a near outage-less migration that probably the safest way.
A. Bring up the new kit at each data center at Layer 2 only so your Vlans are available on the new gear for your end point migrations
B. Migrate everything over
At some point you are going to need to move the SVIs (Subnet Gateways) down to the new Layer 3 device. This means everyone gets a new gateway Mac or you hardcode the old MAC on your new L3 device. Usually, the devices can take that brief hit so you don't wind up with a hardcoded MAC in your configuration but much depends on your tolerance for even a brief outage.
At some point you will need to bring the "North bound" stuff down to your new L3 devices. For that, and again much depends on your configuration, Id make sure once your data center 1 DIA is down your traffic will re-route over to your data center 2 DIA. Or you can update your routing to send all outbound traffic to the other data center. Once you re route the traffic, bring the DIA links down to your new device. Rinse and repeat at data center 2.
And you will need to swing the interlinks.
The order in which all of that needs to take place will be driven entirely by your current logical configuration but those are the things you will need to consider.
