Sure, its okay.
Our team tried to enter two robots at IRI last year, building the second one during the summer (actually, it was a modified version of the 2002 robot, which we didn't like much, so we decided to make it a stack destroyer). Although we failed, it is perfectly okay to modify the robot during the summer for off-season.
This year, I hope to build one during the summer. The second place idea
(my idea
) during the design process was to have a robot with two huge arms and an attaching mechanism at the top. The arm could not only cap big balls, but could also store them between the arms. It could store one while capping another, or even while hanging. It would also have a radial lift mechanism that would attach to the bar and use the strength of the arms to raise it to the platform and hang autonomously. Its arms would curl, and then hook it up to the attaching mechanism. The attaching mechanism would be on a turntable, and would have a wheel to move around on the bar at the top. At the top, it could prevent robots from hanging, block enemy goals, cap its goal, or transfer balls from one goal to the other. Although I doubt it will get approved, and doubt even more that it will get finished before June 18, it is a pretty cool idea nonetheless.
Anyways back to your question, yes, you should definitely improve the robot this summer, if you are able to do so.