The way around it is to stop growing the DFZ routing table by the size of the Prefixes. If customers could have PI addreses and the DFZ routing table was based, instead, on ASNs in such a way that customers could use their upstream's ASNs and not need their own, then, provider switch would be a change to the PI->ASN mapping and not affect the DFZ table at all.
One way to do this is for two ISPs to band together in order that each ISP can sell half of a joint multihoming service. Each ISP would set aside a subset of their IP address space to be used by many such multihomed customers. Each ISP would announce the subset from their neighbor's space which means that there would be two new DFZ prefixes to cover many multihomed customers. Each multihomed customer would run BGP using a private AS number selected from a joint numbering plan. This facilitates failover if one circuit goes down but doesn't consume unneccesary public resources per customer. This does require the two ISPs to maintain a strict SLA on their interconnects in order to match the SLAs on their customer contracts. The interconnect then becomes more than "just" a peering connection, it also becomes a mission critical service component. Of course, the whole thing multihoming thing could be outsourced to a 3rd party Internet exchange operator with some creativity at both the technical level and the business level. The IP address aggregate would then belong to the exchange. More than 2 ISPs could participate. Customers could move from one ISP to another without changing addresses. The SLA on interconnects could be managed by the exchange. Etc. --Michael Dillon