Hello @FabrizioQuattro
I have to revise my statement.
Since it is an SBB long distance ticket, the conductors can amend the ticket. But, you must actively seek help from them, contact them before getting on the train at GVA; at Geneva Airport, they are usually standing in front of the train until departure time.
The ticket can be amended by the conductors, to be valid via Lausanne; the price difference is +2 CHF. It is, however, not possible to change the destination, only the via can be changed.
To pay CHF 2 more (and to use it partially) is still cheaper than purchasing a complete new ticket from Morges to Lausanne (CHF 7.80) or an eventual ticket cancellation fee (CHF 10) and a repurchase.
Kind regards,
Roland