It'd be interesting to discuss the reasons why, since it's a fairly implausible decision to make on Russia's end unless there was something so devastatingly important going on at the same time that they had to leave Serbia. Remember Russia's pretty much the self-proclaimed defender of Slavic peoples and that had a lot to do with their declaration of war on Austria-Hungary, if they abandon Serbia they'll be seen as an unreliable ally for other nations, especially in the Balkans where if the Austrians win they'll be stronger, combined with Turkish possessions there and the Russians are going to pretty much be out of the Balkans.
Well, let's say that Nicholas II was aware that russian armament proces isn't yet over ( it was targeted to end in 1917 ) and that Russia isn't ready enough for major war.
Your best hope for this is for Rasputin, who was deeply opposed to the war, have even more influence over the Tsar than OTL.
A Russian "revolution" under these circumstances would most likely be very different from the February Revolution of OTL. The Tsar would be deposed and replaced by his Francophile uncle.
True, but that still means that Serbia flips a bitch at Russia for leaving them high and dry before the Austrians.
Also Russia's consideration was initially that they were going to be fighting A-H and nobody else, Czar Nicky's declaration of war on Austria rested in some degree of confidence in that particular consideration. Even without full rearmament, Russia kicks the crap out of Austria if it's just those two plus Serbia duking it out.
A Russian "revolution" under these circumstances would most likely be very different from the February Revolution of OTL. The Tsar would be deposed and replaced by his Francophile uncle.
With Grand Duke Paul Alexandrovich?
Maybe not by a coup, maybe another "revolutionary" bomb?
For all their size and supposed power, A-H's army was pretty inept by then. Given any number of exceptional commanders and some sheer dumb luck on Serbia's part, they might have been able to pull off a narrow victory (or at least mired A-H in an Afghanistan-style conflict for long enough to reach a stalemate.) Serbia would have been pretty pissed off at Russia, but the Russians have never really cared much for what other countries think of them.
WWI was one of the main reasons for revolution in the first place; there may still have been one, but might have resulted in Nicholas getting the boot and being replaced with someone a bit more competent - the beginnings of a Constitutional Monarchy in Russia, perhaps?