If a war somehow breaks out after all, Serbia will probably act sort of like OTL's Romania: wait some time to prepare and assess the strength of the enemy alliances, then join against Austria-Hungary if the conditions look good.
This would mean: AH can mobilise in 1914 at least 5 Armies at the russian front, and ceep maybe one Army in Bosnia to save the border.
That would mean 5 Armies in Galizia instead of only 3 Armies.
Noone knows, if it would have been enough to win, but it would have been in 1914 more successfull than in OTL.
This would have reduced the pressure on the German 8th army.
One possible Szenario:
One additional Army near Lemberg, and one more army against Poland.
Russia is forced to send the 2nd army to the south, and the 1st russian is now alone against the german 1st army.
The Germans can defend Eastprussia, without Hindenburg ans Ludendorf, ans no regiments from the Westfront have to be sent to the east.
Result for the Westfront: More troops march to the Marne, Germany could have one there, but will be stopped at Paris because of the longer distances and larger logistic problems.
Result at the russian front: larger battles at the AH front, AH can defend Lemberg but the frontier in Poland might be the same as OTL.
Now Beginning of 1915 Serbia looks, if they should join the war.
In this ATL, AH and Germany are of course mor succesfull, because on frontier is missing and AH can concentrate on russia.
Maybe Serbia will continue waiting..
After beeing stopped near Paris Germany will concentrate to the east....