just present your 1K card at check-in (availabilty shouldn't be a problem, it is still low-season in BKK) and they will gladly put you in a (nicer, larger, but no Suite though) Riverside room. This info (in general for all Mandarin Oriental's) was included in my 1K info, back in feb-99 (and it worked twice in sep and nov-99, but I also did present my Leaders Club membership card from LHW).