Really, this is an extension of the Jones/Woods argument. Can you legitimately say the Bills upgraded at WR by letting Woods go and drafting Jones? I'd argue not. The 3rd WR is still, as of today right here in the Jeremy Maclin freaking thread, a problem. If you sign Woods AND draft Jones now your #3 WR is a better player and you have upgraded the WR position. In order to upgrade a position you have to push better players down the depth chart, not just churn. We are not better at CB, we are not better at S, we are not better at WR. We are different, different is not necessarily better and we are certainly not any deeper at those positions.
Bringing it back to the thread, signing Maclin WOULD, however, upgrade the WR position as it would push a better WR to the 3 and thus the 4 and 5 slots.