Actually, Johns himself recently retconned that out in BOOSTER GOLD, with his utter failtastic revisitation of TKJ and introduction of the "solid time" bullshit. That ish starts out with Jim asking Batgirl "I heard you retired" and she goes "nope, I'll do this forever."
Which is a cheap and calculated line of crap introduced for the sole purpose of amping up the tragedy of Booster's failure to save her, poor woobie, OMG let's feel sad for him, BUT... is nevertheless more in character than her giving up the cowl ever was, particularly when seen through the lens of Oracle. So I'm willing to give him that one and let the retcon stand.