The "keep messages" option on a BlackBerry device determines how long messages stay in your messages list on the BlackBerry after they are synced.  It does not affect how much data is actually synced during the activation, nor does it prompt your device to download older emails when you change it.

The default settings on BlackBerry Enterprise Server (BES) syncs up to 7 days or 350 messages during the initial activation of the BlackBerry.  After that, new incoming emails are synced, and stay on the device until you delete them or they expire according to the "keep messages" option. 

It is possible to change the default settings on the BES, however it's a global change that affects all users on the server.  And, more importantly, even if you do change it, the maximum number of days it can be set it to sync is 14 days or 700 messages.