Having recently done an extended parking lot comparison of a 2002 and 2003 Lemans, there are a number of visible changes. Who knows how many we missed being so distracted by the single malt and the red-neck girl being arrested . . .
The front exhaust crossover was new for '03. Also there may be a different voltage regulator. The rear caliper mount is through drilled and the locator pin is safety pinned on the inside of the caliper mount. The tank, fuel pump, filter , low fuel sensor and petcock are all different. Seems like the switchgear changed at some point. For sure the instruments and speedo cable changed.
This may be a bit fuzzy from the presence of the '04 Ballabio which also had some changes . Still, a quick look at these parts will confirm for you which are different from your '03.