In your interviews with the old firm be ready for being asked questions involving
a. Can they count on you only until your next offer of a 15% increase in salary? If longer than that, how long, and why does the same not apply to your current job?
b. What are the reasons you have for moving/staying, and what is the relative weight of each component?
c. Didn’t you consider the inconvenience of the additional commute time before accepting your current job? Maybe your satisfaction with the offered position will be similarly short-lived. Are you capricious?
d. When you were hired at your current position did they have a right to expect that you would not leave immediately upon being trained? Do you feel guilty about leaving so soon? Why or why not? If yes, what are your criteria for being willing to leave anyway? (If not, they may wonder whether you are simply not capable of feeling guilt. Are you a sociopath?)
f. If they make you a formal offer of the amount discussed are you prepared to accept it or are you going to ask for a counteroffer from your current company?