Never, ever. Even the HTC Dream (i.e. the first comercial Android device) received updates without the need of a factory reset. User data is always stored in a separated partition for that purpose.
A software update is always backwards compatible. At least, when no hardware piece has been changed.
Edit: What about a new chipset that’ll be supported by Android 7 codebase? (MSM8975 isn’t, as marvellously explained in this other topic). They maybe have studied the case of a Fairphone OS Marshmallow update supporting both SoCs. This is mere speculation, though.