Flexible Scrum Teams

I am in discussions about how my team can add more agility to our Scrum process. We currently operate a solid Scrum process with two long term teams of 7 per team. My question is what are people’s opinions of making Scrum teams more flexible to swap members in teams each sprint. Change due to need of projects. Breaking into multiple small teams as the need arises. I like the idea of teams being flexible between projects but if anyone has experience with this, how do you deal with Velocity as that usually takes time to settle between teams. Thoughts on this would be appreciated :)