Count: Since we don't know the basis for the change from a Buy rating to a Neutral rating we can only make assumptions. Like you, I assume it is probably related to the increase in the stock price. However, based on the firm's rating criteria I would say that the change was premature.
Playing with numbers, I come up with a little different conclusion regarding potential appreciation potentials. The analysis was dated 20 January, The range of closing prices from Jan 1 - Jan 16 (last trading day before report) was $26.73 low - $28.11 high with average around $27.27. An increase in price to $39 would be $11.73, or approximately 43%. At yesterday's closing price of $33.33, $6.06 (or 51%) of the potential appreciation occurred. Therefore approximately 50% of the original projected potential return was still available.
The reason I use this exercise is that if you use the firm's criteria for rating stocks, unless something else besides the price has changed, a change to a Neutral rating does not seem to be proper. (see highlighed portion below)
"Investment Ratings
Buy - We believe the stock has significant total return potential in the coming 12 months.
Long-term Buy - We believe the stock is an above average holding in its sector, and expect solid returns to be realized over a longer time frame than our Buy rated issues.
Neutral - We believe the stock is an average holding in its sector, is currently fully valued, and may be used as a source of funds if better opportunities arise.
Underperform - We believe the stock is vulnerable to a price set back in the next 12 months."
I will now go into ulterior motive mode. Since according to the report "The author of this report or members of his household have a long position in the common stock of InterDigital Communications, but may not engage in buying or selling contrary to the recommendation." maybe he would like to do some selling at around current prices, but cannot do this while still rated a "buy".
All the above is just my playing with numbers and JMHO.