Hi Friend,

The most important things to remember is to take the backup of complete configurations so that by any chance if it get erased during upgrades you do have the backup.

Also remember not to delete the old image from the box which can retain more than one image so that in any emmergency, box can be reverted back immediately to old image.

Do arrange a proper downtime for upgrades and as per my thaughts start upgrade from APs then to access switch and then to cat4k and finally on cat6k.

HTH

Ankur