In this scene, we see...
But, here, in this next scene we see...
So, is it a big mistake in the movie??
I could be wrong but I believe the character in the top image is shinier, newer (i.e. different) than the one in the bottom image.
During the film, I recall thinking that the character in the top image looked familiar, but couldn't be who you (and I) thought it was. So the next image made sense to me. I had the thought "Oh okay, must have been a different character."
Consider the placement of the individuals in the sending off scene. Front and center are C-3PO and BB-8. Off to the side (and behind another person) is an R2 unit.
One would expect, that if that was R2-D2, he would have been standing in close proximity to the two droids in the middle given R2-D2's importance. That this expectation is not met indicates thats not R2-D2.