Well, given that I finally realized the true difference between Overworld and Sandbox, I can honestly say that I still cannot tell the difference between them based on the games that I have played so far... But, I'll try...
For example(?):
-The first God of War (when you think about it) had a semi-changing Overworld...
-The first Tales of Symphonia (even before reaching the "first" half-way point in the game) is an expanding Sandbox that first comes off as an Overworld... (The second game's Overworld was just a map..)
-Catherine has two Overworlds... kinda...
-Sonic 06 had a Sandbox feel, but it totally acts like a poorly-done Overworld... (The same could be said for Sonic Unleashed... kinda...)
-Sonic Generations has a "great" Overworld... (with a "hidden" trophy room...)
-The later Persona games has a day Overworld used for Social Links, Request, and more... and a night Overworld mainly for choosing your teammates for the night...
-The first Jak and Daxter had an Overworld while the two main sequels were basically Sandboxes...
-The original Legend of Zelda was an Overworld trapped in a Sandbox while it's sequel, Link's Adventure, was basically that only more Overworld than Sandbox...
-All three Hyperdimension Neptunia games had different approaches to an Overworld, with the last two having a similar Overworld feel...
-Kingdom Hearts 2 had a better Gummi Ship Overworld, however the worlds, themselves, in the first Kingdom Hearts were less restricting in overall navigation...
Yeah... I still can't tell the full difference, but both kinda need to have that feel that the world the game creates matters in some way... Otherwise, it comes off as "odd" during gameplay...