Migrating in this fashion is pretty tough because as you add new APs you can cause more interference, co-channel and adjacent-channel.
You could deploy the 2600/5508 first with only the 5Ghz radios turned up so you can migrate the voice over. Making sure all of the controllers, new and old, are in the same mobility/rf group. Also setting the 7925 preferably to 802.11a only but depending on the deployment you may need auto-a, so the phones could drop back to 2.4 Ghz if necessary. Doing 5Ghz first as there is plenty of channels to use.
To minimize client impact, you can manually disable the 2.4 radios on each AP seperately and then go section by section turning off the old and on the new.
Or have a few minutes downtime and switch them all off on the old wlc and on the new wlc. Either way it will take some time for RRM to evenly distribute the channels and power settings for the new APs.
Good luck with the deployment.
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