A change of trains is not needed, regardless of the ticket. If you want to benefit from the Swiss Half Fare Card, you simply have to split the tickets: Venice to Domodossola and Domodossola to Interlaken. A seat reservation is needed for the Venice to Domodossola stretch only.
Compare ticket prices. It might be more beneficial to buy a Venice to Interlaken direct ticket, rather than to split it just because of the Half Fare Card.