Chances are that they will charge a 40-60 euro fee as well, as GoLight U class tickets typically carry such change fee. If they allow it at all, as this 'change' is not described in the fare rules. Call and ask - perhaps they are willing to make an exception.
An alternative method is that you could change the tickets (online) yourself, moving the flights to another day that has S class (or up) availability only. If you would have an acceptable alternative travel day that may work out. It would however require you to wait for the alternative travel day to have availability in class S (or up), and only then do the change. It would also be cheapest to do this to all legs in the booking at once, as you only pay the change fee once in that case.
Depending on the cost of these tickets you could also consider simply booking new ones. Either in class S (or up), or, as Flex in class U. If the current tickets were at a very low sale price that may be the cheapest option in the end. In any case make sure to compare prices for all options.
Do make sure that you first call to verify availability though, like KLouis already mentioned - it would be an extremely sour experience if after all that there is no availability.
Last edited by Xandrios; Sep 3, 2017 at 3:12 am