You can change your bid for base at any time. It will become effective on the next base bid assignment provided that you can hold it with your seniority. There is typically a base bid once a month. It normally takes effect the following month. (It may take a month, but it is not hard to bid out of CLT).

Once you hold a line, you can also SAP down to 65 hours (usually 3 four day trips, and maybe 1 more day to make 65), and then pick up additional trips later in the other base. You can then try to swap one of your trips with a trip from the other base. Some guys are based in one city, but half of their trips start in another base.