After I've made a pull request, can I change the branch?

Question

git

After making a pull request (not confirmed), is it possible to change the branch? What will happen?
1. Add new commits
1.1. If you make the next pull request, will there be a difference with master, or with the previous pull request? Can't do the next one until the previous one is confirmed?
2. Merge from master, and remove the branch