Hi,
the steps you need to perform for changing the AIX-level are described wihtin this pdf: http://www.pdc.kth.se/doc/SP/manuals/vac-6.0/pdf/en_US/getstart.pdf
(page 32).
yes, you need to install it again on the 5.1-system, just moving files does not work since there are some OS-level-dependent...