Ron thanks for the prompt reply
Bit of controversial here , I have been advised , that for safety , I need to upgrade the AP's as I upgrade the controllers.. So basically move all the aps's ( 800 plus ) to ha controller .
Upgrade original , move aps back so they upgrade to 9.21.18 .. then on the now emply controller upgrade to 9.21.18 , upgrade to 10.41. X then move the AP's back , which will upgrade them , now aps are all at 10.41 ... finally upgrade original controller , and move AP's to the correct c5210 unit..
Phew !!! that took a bit of typing ..
Or basically move aps to HA pair , upgrade to 10.41 , move aps back , forcing ap upgrade , upgrade other controller to 10.41 and then move the AP's to the correct controller ..Less to type so therefore this should be a faster option ..
I need to be a sure as upgrades are possible that minimum disruption and minimum risk , henc ewht I have already been advised to take the long route for the upgrade.