not doing staff change(s) correctly

[Kieren Richard MacMillan]

[Mac OS X 10.4.4 ; Lilypond Version 2.7.28-2 (Build from 30-12-2005  
19:17) ]

Hi, everyone!

Clearly I don't understand the \change Staff feature...  =\
Could someone tell me what I'm doing wrong in the enclosed excerpt?  
It's adapted directly from the docs example, but clearly I introduced  
a problem in the translation!

Thanks,
Kieren.

p.s. here's an attachment of the output I get:

_______________________________

\version "2.7.28"
\include "english.ly"

#(set-global-staff-size 19)
#(set-default-paper-size "letter")

hIa = { \change Staff = harpOneUpper }
hIb = { \change Staff = harpOneLower }

global =
{
        \key e \minor
        \time 6/8
        s8*6
}

harpOneRH = \relative c'
{
    \repeat "unfold" 2 { \voiceOne \hIb g16 \hIa b e b e fs g8. fs16  
e b }
}

harpOneLH = \relative c
{
        \clef bass
        \repeat "unfold" 2 { \voiceTwo e16 fs g fs g a b \hIa e \hIb b a g fs }
}

\score
{
        \new GrandStaff
        <<
                \context Staff = harpOneUpper { \global }
                \context Staff = harpOneLower { \global }
                \context Staff = harpOneUpper \new Voice
                {
                        \harpOneRH
                }
                \context Staff = harpOneLower \new Voice
                {
                        \harpOneLH
                }
        >>

        \layout { }
}