I don't have a nose piercing, so I can't comment from experience, but many body piercings that aren't really ready for new jewelry can collapse when you take the original jewelry out, meaning the passageway of the piercing closes down partially or totally.
I've had that issue with ear piercings. The main thing is to not force the piercing or try to hard to put in new jewelry, as this can lead to trauma to the skin, and scar tissue forming.
If new jewelry doesn't want to go into a piercing, put the old jewelry back in, and try again in a few weeks. Also, if the new jewelry is thicker than the old, you may not get it into the piercing without a professional stretching the piercing for you.