Keep your commitment to the new employer, it's important. Possibly offer to consult for the previous employer, and/or simply let them know they can contact you

Keep your commitment to the new employer, it's important. Possibly offer to consult for the previous employer, and/or simply let them know they can contact you if there's something they can't figure out. Don't expect to be contacted -- they have 14 other developers which is far more than many projects that people leave.