The conditions vary from Company to Company there is no general answer :/

Usually you can exchange reservations on SNCF (French Rail) services before 1st day of validity (means atleast one day in advance) but note that on some trains there are just limited seats for Eurail user and it could happen that the new desired train the Eurailseats are already sold out.

Going by what Belgian railways publish about the exchange conditions:

Domestic reservations in France: can be exchanged or refunded before the booked departure. When exchanging, there must still be seats available in the new train of course.

Domestic reservations in Italy: reservations of 10€ or less cannot be changed or refunded (this basically includes all day train reservations).

It is likely that that also applies to reservations for France and Italy bought elsewhere, such as from Eurail or ÖBB.

Thanks to all, looks like I will have to lock in reservations from Catania to Paris via Italy, and on to London.

Rupert