ONE person's misfortune can bring another an opportunity of a lifetime -- and that is just what happened to Pia Morley.  The 21-year-old will take to the stage on Friday when Singin' in the Rain sweeps in for its Brisbane premiere.     Morley will make the transition from understudy.     She will play the key role of Kathy Seldon in the David Atkins stage adaptation of the much-loved Metro Goldwyn Mayer musical.     Her big chance comes at the expense of Rachael Beck, who quite literally broke a leg.     The actor was driving through Melbourne's rain on Friday last week when she had an accident, sustaining a knee injury. NOTE: I saw Pia perform the role of Kathy Selden when I saw the show in Sydney -- she was fabulous!  She was on because poor Rachael was unwell.