This is exactly why it is such a bother that AY no longer displays the full ticket rules during any time while booking. At least I have been forced to get them all from expertflyer and don’t even know if I could get them from AY without separately contacting CS.

In a way I can understand why upfare from flex to classic or light is not allowed, as it might create weird situations where in would for instance book Y flex and upfare to J light, thus ending up with a J light ticket where a substantial part of the the fare would be refundable due to the underlying J light ticket. This rule should be clearly visible to the customer, however.