No. jsp:useBean does not reasign values. It declares a (bean) id and associates it with an actual bean. If the bean does not exist, then it creates it.
So, in the case the bean already exists it is just a declaration, so that you can use jsp:setProperty and jsp:setProperty.
The spec puts it this way:
A jsp:useBean action associates an instance of a Java programming language object defined within a given scope and available with a given id with a newly declared scripting variable of the same id. When a <jsp:useBean> action is used in an scriptless page, or in an scriptless context (as in the body of an action so indicated), there are no Java scripting variables created but instead an EL variable is created. The jsp:useBean action is quite ﬂexible; its exact semantics depends on the attributes given. The basic semantic tries to ﬁnd an existing object using id and scope. If the object is not found it will attempt to create the object using the other attributes.
[ April 27, 2007: Message edited by: Sergio Tridente ]