Lets say I have a 3750 switch and it was successfully discovered and inventoried etc.
If I then swap out that 3750 with a different model of switch (keeping the same hostname/IP/config etc) then what is the best way to update CiscoWorks with the new chassis details etc?
Do I have to manually delete the 3750 from device manager and re-add/discover it ?
Unless the type is the same (i.e. one model of 3750 with another model of 3750), you should delete the device from DCR, and then re-add or re-discover it. The reason for this is that the inventory and configuration (i.e. hardware/interface data) will be different.