didn't he... (spoiler)
...switch out the suitcase?
What was the point of even including that if it didn't play out in any way in the movie?
He took the bomb out of the smaller case, and placed it into the larger case.
The reason why he didn't just switch the ribbon to the larger case is b/c the villain would probably realize the weight was off. Additionally there is a tracker on the bomb that the villain can see on his phone.
And, finally you may ask why didn't he just disable the bomb right then?
This is because he barely was able to do it last time, and needed a long poker type item to press the disable button. This process may have taken longer to find that special item in that cargo area and press the button than just simply swapping the bomb into the larger case.
His plan was that by swapping it into the larger case, it would not fit into the overhead and could no longer be considered a "carry-on". That would then require it to be scanned, and then they would know which flight he was on and could disable it later.