While Fools should generally take the opinion of Wall Street with a grain of salt, it's not a bad idea to take a closer look at particularly stock-shaking upgrades and downgrades -- just in case their reasoning behind the call makes sense.

What: Shares of Sirius XM Holdings (NASDAQ:SIRI) slipped about 1% this morning after Wunderlich Securities downgraded the satellite radio company from buy to hold.

So what: Along with the downgrade, analyst Matthew Harrigan lowered his price target to $3.80 (from $4.20), representing about 8% worth of upside to yesterday's close. While traders might be attracted to Sirius' potential short-term upside, Harrigan thinks that a sale to Liberty Media (NASDAQ:FWONA) is becoming less likely and should therefore be valued on a stand-alone basis.

Now what: According to Wunderlich, Sirius' risk/reward trade-off is pretty balanced at this point. "We now apply just a stand-alone value for SIRI as we are skeptical that there will be a material sweetening of the 0.076 exchange ratio (after a 2:1 C stock dividend) on the Liberty Media (LMCA-$129.86, Buy) bid," noted Harrigan. "Our stand-alone target had been $4.00 vs. $4.30 with a deal, but we are now allowing for a slightly lower 1900 S&P 500 assumption on our market-linked valuation, along with a $120 SPX earnings assumption that bumps up the cost of equity and constricts the 'as is' target to $3.80." Of course, with Sirius shares now off about 10% over the past month, it might be a good time for contrarians to grab the built-in optionality.