Thanks to Mike’s comment, I’ve re-read the doc and I’ve realised that my current user (i.e. userA that already has the create privilege) wasn’t a direct/indirect member of the new owning role…
So the solution was quite simple – I’ve just done this grant:
grant userB to userA;
That’s all folks 😉
Update:
Another requirement is that the object has to be owned by user userA before altering it…