[plt-scheme] upgrading installed planet packages

Hi,

I track PLT using svn, re-building it from source every now and then,
which is a rather painless procedure. I usually choose the in-place
build/install, linking the binaries to somewhere in my PATH. But
everytime the version number of PLT changes, all my downloaded planet
packages (living in ~/.plt) are forgotten, because they live in
version-specific folders. Is there away of automating their
re-installation after an upgrade?

I've got the feeling that this is a FAQ, in which case i apologise and
will be happy with a link to the relevant discussion or document.

Thanks a lot!
jao