This paper presents a model for analysis of changing wellbore storage during well testing. Our model is based on a modification and extension of a model for wellbore phase redistribution. The result is a general solution in Laplace space that can be used to add changing storage to a variety of well/reservoir models. Increasing and decreasing storage cases can be accounted for analytically. We present several field examples to show the application to well tests with changing wellbore storage.