Changing the Reference type of a stationglobal

I am creating a Stationglobal variable that is a number. Teststand by default creats this as a double precision 64bit floting point and I need to make it an unsigned 64bit number.

I use this as the way to create the variable:

StationGlobals.NewSubProperty("Station_Equipment.DMM.Session_Number",PropValType_Number,False,"", PropOption_DoNothingIfExists)

Which make the double precision no problem. I then use this to try and change the type reference to unsigned 64 bit:

StationGlobals.SetPropertyObject("Station_Equipment.DMM.Session_Number",PropOption_DoNothingIfExists, (PropValType_Number.Type.Representation, PropertyRepresentation_UInt64))

This does not seem to change my variable. Any suggestions on what I am doing wrong?