Decommissioning devices

At the moment I am moving my itunes over to a new purchased imac from my old PC, I am unable to do this as I am being repeatedly told that I already have 5 devices asigned to this apple account and I must decommission one of the devices.

I have been on to my itunes account to decommission one of the devices and the only option I can see is decommision all devices.

I am worried if I do this i will lose all the other divices, I pad, iphone etc.

Can somebody confirm if this will happen or a better way to do this?

Thank you.