Before the girls are helping Rachel pack, we see Monica in a white button down top w/ a red skirt. They are in Rachel's room & going over what they're going to miss. They go to a commercial & when it comes back on, Monica is in the same position as before but now she has a purplish top with jeans. Wow!
I just saw this episode a few days ago, and I think what happens is Monica is in the first outfit when she finds out Rachel hasn't packed yet and the three of them decide to pack together. Then, when they come back from commercial she's changed, because before she was dressed to go to dinner, and she would need to put on something more comfortable before getting started on the packing.
It was also probably more convenient to put her in pants. Maybe they'd filmed the scenes with her in the skirt first and then realized that she would have to be running around the table. She wouldn't have been able to run as fast (running around a small circle is harder than it seems) in the skirt.
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