As seen on this "Space Shuttle Program Development Roadmap":

http://spaceflight.nasa.gov/shuttle/...es/roadmap.pdf

....the number 1 goal of shuttle upgrades was "Fly Safely", and the
very first safety upgrade on this schedule was the MMOD/WLE fix which
was to strengthen the Wing Leading Edge RCC panels.

This one slide gives an excellent snapshot of upgrade priorities circa
1997.

(Obviously, these priorities were subsequently rearranged.)


~ CT