Adding a DataMember to an existing DataContract in WCF

I would like to add a DataMember to one of my DataContracts. I would like to know how existing servers and clients will behave in the presence of a new DataMember if one of the parties isn't updated.

I recall there is a way to make the DataMember optional, but I wonder if it would work in all scenarios:

• updated Client => old Server
• old Client => updated Server
• updated Client <= old Server
• old Client <= updated Server