Sorcerer/Wizard: Can I dismiss my familiar so I may select a new familiar?

This isn't addressed in the rules anywhere, but yes, you should be able to dismiss a familiar if you want to select a new one. However, you must still wait 1 week and pay 200 gp for the 8-hour ritual. Dismissing a familiar is ending a link between your soul and it, so it should probably take about an hour.

The exception to the above is if you take the Improved Familiar feat, which allows you to immediately replace your familiar with the new familiar, at no cost or time required (it is assumed this occurs during whatever preparations you make while leveling up).

Once dismissed, an animal familiar is just a normal animal of its type (a special familiar from the Improved Familiar feat reverts to a normal creature of its type). Whether or not it wants to remain with you is up to your GM and probably based on how you treated the creature while it was your familiar.