I had the exact same prob so had to phone them & do it seems ludicrous in this day & age of technology...
I also wanted to change one of my flights into husband name but every time I tried it auto changed both legs of the journey to Derek’s name, so the operator on phone suggested exactly what Keith has said re the luggage wait until the outward has elapsed then do it, again I found this to be really ludicrous, maybe it’s the same with other airlines too, tho I’m sure Iv booked a bag one way on Thomas Cook website in the past