When you say the switch is in client mode, what do you mean by it? Is it VTP client?
If the switch you are upgrading is not the STP root, the outage should be nil to minimal(there will be some flooding when the link between the switches goes down and up since it generates a STP TCN).
If it is the STP root, another switch in the network will have to be elected as the root and will involve recalculation.