That's what happened to me last week! We played Shostakovitch cello concerto. I've played it before and had all the solos ready. I was sure it would be a walk in the park but it turned out that the tempo in the first movement was much slower than what I was used to which can be very uncomfortable sometimes. On top of that, the conductor made me sit next to the piccolo which is not where you want to be! I wasn't very happy at the end of that rehearsal.
When the tempos are so far from what you've practiced, it can make you miss notes and it will take a bit of time to adapt your playing by breathing at different places and changing some fingerings if needed. You might want to practice at different tempos before the first rehearsal just in case. Obviously, I hadn't done that this time! With all the things we have to learn, we don't always have time to learn them at different speeds. Usually, I practice the music slightly faster in case the conductor speed things up but that's about it.
So, first of all, I told the maestro that I just couldn't sit next to the piccolo! He understood and let me sit behind. I brought the music home and relearned the piece slower with different fingerings and breathing places just in time for the dress rehearsal and the concert which went quite well.
You don't always need to have a plan B but if you have a big solo to play. It's a good idea to have it at various tempos in case the conductor wants it much slower or faster. In any case always be ready to adapt. You never know!