Need advice on best way to cut over to new network

I agree with your suggestion. If you want to eliminate physical work on cutover, run the cables and plug them in, but disable them on your core only. On night of cutover, shutdown the old core's interface, and bring up the new core's interface. That way if you need to back out, you can just reverse without worrying about losing connectivity to the IDF switches (at least in theory). Do not, and I repeat do not, turn up the new core with same vlans and ip addresses prior to cutover or you will have problems. Well unless of course your new core is connected to your old core and has all of your LAN/WAN routing connections. (IE, your old core is .2 and .3, and your new core is .4 and .5 with all the same HSRP group, etc). I'd just avoid that to be safe if you can avoid it.......

Thanks for responding. I have 13 IDFs with 3 crossover cables in each, that's 39 ports to disable/enable. Is there an easier way to limit the traffic? I used an allowed vlans command at the core, could I just disable those vlans (except for management) and stope the traffic that way? Any ideas? thanks

After re-reading your first post, I think I have a little bit better of an idea of what you are doing. So if i understand correctly, you have a completely new layer2/3 core, and new IDF switches that sit next to your old IDF switches, that currently run back to an old core. I would consider turning up the trunks between your new and old IDF switches, and keep the ports blocked at your new core. On night of cutover, I would disable your old core's trunks to the old switches, and enable the new core's trunks. You would technically be moving from old core -> old idf -> new idf X -> new core to a configuration that is new core -> new idf -> old idf X -> old core. Providing your configs are exactly the same, this should work presuming your trunks are all identical on all these trunks (existing and new).

Personally if you can prune back VLANs that don't need to go to an IDF, you should do so, but it is not a prerequisite and you can always go back and prune later. However, I would strongly encourage you to completely shut down the old core on cutover, and if you are going to use both the old and new idf switches long term, each of those should be homed to the core, not daisy chained off one another. In short, and in my production world we use 4 gig etherchannel, but one fiber gig link from core 1 to idf 1a and idf 1b each, and one gig fiber link from core 2 to idf 1a and idf 1b. I would not only prune back VLANs you dont need, but I would make Core 1 own layer2/3 (vlan/hsrp priority) for odd vlans and Core 2 own layer2/3 for even vlans. As I said above, these can be changes for another day, but I would caution you against keeping a 'square' architecture.