I took admission in AI R1 and didn't get upgraded in AI R2

I took admission in AI R1 and didn’t get upgraded in AI R2. So i don’t need to join in that stipulated time (26 oct) right? I contacted the college they said i can join lately although tomorrow is official joining date.

The institute is right. 20th is just the earliest course commencement date. You can join later too if institute is ok and your course would be three years from that date.