Upgrading 3.0 to 3.1 ! Are there special instructions ?

[Connie]

As a result, I would expect the old config file to continue to be active.

the old gsconfig.php will work, however there are some new entries in the config which will use the default values, so you have no problem to continue with the old config-file

However, I suggest you to read this information on gsconfig.php:
http://get-simple.info/wiki/config:gsconfig

all new values are noted (example:
GSNOCSRF (3.1+) allows you to turn off the CSRF protection system )

so you can see which options are new and which of these you will need