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Summer internship/full-time offer question

I will be starting my internship this summer in advisory for a big 4 firm and will specifically be working in internal audit. I am currently on track to graduate in June 2016 at the end of my 3rd year of college. Graduating a year early was my plan at the time I applied for and accepted my internship offer; however, over the past few months, I realized I don't really want to graduate early anymore. Instead, I now plan on pursuing a minor or two and maybe even studying abroad, which would push me back to graduating in 4 years. In the event that I receive a full-time offer at the end of my internship, would I be able to defer it?

Furthermore, since I'm not intending to graduate until 2017 now, I want to go through internship recruiting again in the fall to pursue a more finance-heavy internship for next summer such as corporate finance or investment banking. I really feel like I rushed into this internship and that I can land something better. If I want to interview for other positions in the fall, would I have to first decline the full-time offer, assuming I get one? How long do you usually have to accept a full-time offer after an internship? I've heard that you should never reject an offer unless you have something better lined up, but in this case I would be rejecting a full-time offer to pursue a summer internship. 

My internship hasn't even started yet so this could all change, but I'm just trying to see if anyone has any opinions regarding my dilemma. Thank you in advance.