Number of instances is specified in cscfg file. So I dont think that would cause this error. This error would occur, if you have difference in number of end points for any role in your csdef file between production and staging. Can you doubel check this?
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It has not been possible to change the number of endpoints in a deployed service - until today. Microsoft has just
announced- like 2 minutes ago - the possibility to do an in-place upgrade changing the number of endpoints. However, you can't change the number of endpoints in a VIP swap - presumably because a VIP swap only swaps the internal IP addresses (and doesn't
create new public IP addresses.)