This is incorrect and the agent with whom you spoke was absolutely wrong. Your fare rules should have a "guaranteed airfare rule" clause. Refares should be calculated from the base fare only, not counting any vouchers (whether Type A, B, or AB) or e-certificates. In other words, if the base fare drops, you should receive the difference between your original itinerary and a refared itinerary priced using the new (lower) base fare and any vouchers/e-certs applied to the original itinerary.

(All T&Cs for vouchers/e-certs that I've ever seen also state that the voucher is not applicable to a new ticket except when exchanging for an identical itinerary, e.g. when refaring.)

You should call back and, if the CSR balks again, ask him/her to look up the "Guaranteed airfare rule" in your fare rules... and to ask a supervisor if they require further clarification.