[plt-scheme] moving to PLT Scheme 207

>Assuming the collections still work with 207, you can move them over,
>remove the compiled subdirectories, and run setup-plt to recompile
>them.  (And having a .plt file makes this unnecessary.)

Thanks -- this is about as much work as just retrieving the .plt files
(some of which may have been updated themselves in the meantime) and
running setup-plt on them. So what this tells me is that I should keep
downloaded .plt files around in a directory, and when a new version of
PLT Scheme is released, I can upgrade and then reinstall all the extra
collections from that directory. --PR