Below steps help you with minimum downtime if the configuration correctly configured and tested before Migrations.
Prepare offline : (upgrade to latest code 17.9.4a or above)
1. Prepare new Cat 9500 inline same configuration as Cat 6500 (some of the configuration need to change for IOS XE code)
like some example i used :
https://www.cisco.com/c/en/us/support/docs/switches/catalyst-3850-series-switches/118629-technote-qos-00.html
3. Map the ports Cat 6500 to Cat 9500 Moving Physical links and collect all the MAC address on cat 6500 this help you when you looking to troubleshoot, including show ip arp
3. create VLAN and SVI on the new Cat 9500 (put the SVI in shutdown model) - at this stage the switch act as Layer 2 only
4. use free port to connect between Cat 6500 to Cat 9500 (use Layer 2 port-channel) - make sure Cat 6500 still root bridge for the all VLAN and gateway still on cat 6500 only for now.
5. Prepare the Cat 9500 bringing online and check the connection between Cat6K to Cat 9K
6. may be use test laptop and connected to respected VLAN check all the services can able to access with out any issue.
7. On the maintenance window - Move One SVI from old Cat 6K to Cat 9K ( check with users all working as expected,) (in case required you may need to clear ARP on Cat 6500 if they stuck for long - soon you shutdown the SVI)
8. Once all the SVI moved to Cat 9K - Move to Firewall Links to Cat 9500 - check all services are ok.
9. then move rest of the downlink ports connection to Cat 9500
10, try move STP root to cat 9K
11. check all working as expected (including routing)
12. turn off the link between Cat 6500 to Cat 9K and monitor
Hope this help you.
