I'm pretty sure that POSIX mv will do the trick, although you will want
to be sure that the accounts and groups exist as you want them first, or
you will get HFS directories on your system volume set, which are a bit
trickier to move. Also, since POSIX depends on the shell to parse wild
cards, whereas most every other operating system in the world allows the
individual command to parse wildcards, you will need to be in the shell,
assuming the existence of more than one file to move. I have no idea how
mv affects ownership, so you may need to deal with that as well.

moving a group:
shell/iX> mv /ACCOUNT1/GROUP1/* /ACCOUNT1/GROUP2/*

moving an account:
shell/iX> mv /ACCOUNT1/GROUP/* /ACCOUNT2/GROUP/*

And of course, you may or may not have MPEX, which probably does this
beautifully; arguments can be made for either, but the ones I can think
of before coffee apply to MPEX and POSIX equally, such as any given box
may or may not have either one.