We will separate your questions to answer below.
Will current sites created in V6.2 be imported for V6.4?
=> Yes, the current websites created with RVsitebuilder 6.2 will stay even after your RVsitebuilder updated to 6.4.
Will sites created in V6.2 work in V6.4 or will they have to be recreated?
=> Every project from RVsitebuilder version 6.2 will be able to continue to edit in RVsitebuilder 6.4, and also use new features that added in "Page" tab.