That would worry me personally. If I were you I would call KLM or the travel agent you bought the ticket from, check that the whole itinerary is indeed confirmed and ask them to pre-assign a seat on your return at the same time. As said by others, you seem to have everything working for you (long haul, less than 30 days to flying etc) for it to work. Of course it is always possible that all pre-assignable seats have indeed been assigned (some seats are kept for on the day assignment so it might happen if the flight is fully or over booked) but it sounds rather unlikely.