Of course I can't speak for the OP's specific flight, but I've been told it can take a good 2 hours to unload/reload an international widebody once the decision is made to swap. Add in replacement crew issues and you could quickly get to 3 hours.

Saw that play out on a TATL flight out of ATL a couple of years ago when after a series of attempted fixes they decided we needed a new plane. We cut it very close to crew time out, at one point the 3 pilots were behind a closed cockpit door not talking to anyone (FAs, tower, gate) and FAs were standing at the open cabin door looking at their watches. I was traveling with a group and there was a DL guy in the mix, his colleagues kept texting him for updates and all he could say was, 'the cockpit door is closed, if they're not communicating with you do you want me to go knock?' Another guy in our group was chatting with the crew loading catering & the DL guy kept telling him to sit down and stay quiet! In the end, we went with the original crew and got out just in the knick of time. We were all very relieved once the wheels were up.