Applying to Medicine using past results

[tsesoyum]

Hi guys, I'm not sure where exactly this topic should go, but I hope to find out a little more from people who have experience with similar situations!

General question: Will local/overseas unis accept applications in 2019 that use A level results of 2015?

More specific to my context: I'm currently serving NS and will only complete it in late 2017. I'll be doing NUS Dentistry, but in the event that I don't find it suitable for me (after taking it for a year or two) is there still a chance for re-application in 2019 and beyond, to a local uni or overseas uni for Medicine?

Further details:
I have applied to both NUS and NTU Medicine but wasn't accepted for the 2016 intake. What are my chances for re-application in 2019, using 2015 A level results?