If you accept the seat in Round 1 and choose the "Accept and Upgrade" option, you're eligible for upgradation in Round 2. If you again choose "Accept and Upgrade" in Round 2, you will be eligible for Round 3 upgradation.

However, if you don’t get a better option in Round 3, then you’ll have to join the seat allotted in the last round where you accepted.

You cannot go back to previous rounds or upgrade again after Round 3.

So, yes — joining the last accepted seat becomes compulsory if no upgrade is received further.