Let's look at the the changes made for the 2010 model year

:
- Beefier arms and shoulders
- New stomach design
- New hips and thigh design
- New chestplate design (as evidenced by on-set photos, such as the one of RDJ sitting in the donutshop's donut)
- New gloves
Most of those changes look like proportional, dexterity, or cosmetic changes. For example, the new arms balance out the physical proportions, where as the arms of the Mark III always had a stubby quality to them. The new thigh, stomach, and hip area are more and more visibly segmented, for easier movement between the plates. The chest repulsor looks moved up in relation to the chestplate, so it
finally sits above the actual location of Tony's chest reactor (the Mark III's was a bit too low in comparison, though they fudged it together in the suit-up sequences at his house.)
As for why the Mark I is suddenly put back together, it
is Tony Stark, afterall, and he apparently had
digitized plans of the suit on hand when he was rescued, even though he had what looked like a '90s laptop (with DOS

) when he built it with Yinsen. It's not as it all the pieces weren't in the box to begin with (even though they shouldn't be there.)