My reply is a compilation of what has been said, plus a question!
You cannot force the website to do this; it may, however, do it automatically if the return sector has no more K fares available.
Presumably you want to do this to upgrade the return -- in which case you need to phone VS anyway, so you might as well call to start with, because then you can book exactly what you want in one fell swoop with no subsequent change fees.
And that leads to my question -- you would only want to do this to try to upgrade, right? For a moment I wondered if you wanted to have more flexible conditions on the return sector, but then I remembered the rule that says that the most restrictive conditions on the ticket apply to all sectors.