What is the license count on each controller currently?
How many APs are terminating on each controller?
If there are enough licenses to accomodate all the APs on 3600 controller then you could follow the following procedure
1. Create a AP-group on the M3 controller with AP system profile LMS pointing to IP address of 3600 controller
2. Create the exact same group on the 3600 controller
3. Re-provision the APs on the M3 controller to the newly created group and set the "Master" IP to that of 3600 controller
4. Once the APs have moved to 3600 controller,
a. configure the M3 as a local controller to 3600 (masterip.....)
b. Configure the 3600 as master of M3 controller (localip ......)
5. Step 4 will require a reboot of the M3 controller only
6. Once M3 controller comes up it will sync the configuration from the Master controller i.e. 3600
7. Now, change the LMS IP to that of the M3 controller under the AP system profile to move the APs to the local controller.
Hope it helps !
Regards,
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HT