M.M.
Been there, done that! This same thing happened to me 5 yrs ago. I was working full-time and looking for part-time. Was offered a job doing the exact same thing the hours and days I wanted. I approached my boss, he said let me talk to the Pres. Next morning the other Co. calls me and says they have changed their job description to full-time.
SOOOOO I went into work that morning, let my boss know what happened BUT I also said to him that I was searching for a new job ONLY because I really needed a part-time schedule not anything against him or my current job. I voiced that I loved my job tremendously and only hoped this would not cause any waves, per se. Well he said he would still look into seeing if they could meet my needs and sure enough they did! They liked me so much that they didn’t want to lose me. I’ve always been 100% honest with my boss which is one of the reasons I’m so valued within the Company. That is why they did what they could to make me happy.
Good luck with however you decide to handle this!