i get the feeling there is some confusion here about what these "weights" are and why they can be changed.
As I understand it - and correct me if I am wrong, it works like this...
Mr Boing states a structural MTOW. That can never be exceeded and I'll eat my hat if RYR does that.
RYR pay en-route charges to Eurocontrol based on a declared MTOW which is not a structural limit but a bureaucratic one which may be some lesser figure that they declare to Eurocontrol and is included on the repetitive flight plan (?) Matters not, it is declared for each airframe. (Think of it as a Flex MTOW) This is to bring that aircraft into a lower weight category to reduce en route charges as described above on shorter sectors where most of the fleet operates well below structural MTOW. The loadsheet will reflect this and state the reduced MTOW. As far as the crew are concerned that weight is the MTOW, it means little to them whether it is the structural or the reduced one. As far as the crew are concerned this goes no further than saying, "Ah. I see we have a heavyweight today" or - sometimes - "Uh oh. They've given is a lightweight for Alicante..."
In the event that the lightweight is allocated for a flight that needs more capacity usually a phone call to ops gets a new PLOG at the structural limit and the crew is happy. NO limits are exceeded, but the company must declare the change in weight to Eurocontrol so the correct en-route charges are levied.
It is easy to see how an unscrupulous operator might "omit" to declare that he is operating actually high weight aircraft when he has declared them all lightweights...I gather this is what is being suggested. If so it is a bureaucratic fiddle - nothing remotely safety related beyond indicating a worrying attitude to rules.
And I think we all know what the IAA are likely to do about that. I suspect (ha!) the DFS are rather more punctilious and Eurocontrol will probably not let them get away with it either. Time will tell.