As far as Syria goes there was never any attempt at 'regime change' as such
But a more concerted effort to support the moderate opposition would have at least given Assad and Putler something to think about
None of the regime changes that did occur in the Arab Spring have gone very well. Look at Egypt as well as Libya. Yemen. Even Tunisia, which is stable, reports horrifying torture by the govt, although it's a stable country still
One could criticise Obama for backing uprisings there, but imagine the fuss had he continued to side with Ben Ali and especially Mubarak, Gafdafi or indeed Assad. All of these countries already had revolutions, which threatened and indeed created massive upheaval. So 'regime change' there was already on the way. The one place with the ancien regime still at least partly functioning is the worst of the lot