For cap and grow to be viable, the two iterations of the technology must be able to operate simultaneously. There is no requirement for a flag day in which a complete migration must be performed. Side-by-side operation could be enabled by a translation mechanism, a gateway that unites the two technologies, or a segmentation in which the technologies operate independently.
For example, enterprises have long relied on time division multiplexing (TDM) circuits for WAN connections. Fractional T1s, T1s, DS3s and OC12 circuits are examples of TDM circuits.
TDM WAN has numerous disadvantages: TDM provisioning is slow--the time to obtain or upgrade a TDM circuit is often measured in months. Upgrade increments aren't granular. The next upgrade for a DS3 circuit (45 Mbps) is an OC3 (155 Mbps). Using multiple DS3 may be an option for bandwidth in between 45 and 155 Mbps, although discrete circuits can have repercussions on load balancing.
While TDM may dominate your enterprise WAN circuits, Ethernet is the technology of choice in the LAN. The network builds in the last eight years have resulted in a sizable Ethernet footprint for the U.S.'s top metropolitan areas. Ethernet is quickly emerging as the preferred WAN access technology. Ethernet cards for your WAN-facing routers are inexpensive. These circuits can be upgraded in small increments, and these upgrades usually involve only a few keystrokes by the provider. No more waiting months for bandwidth upgrades.
How might a cap-and-grow strategy work in an enterprise moving to Ethernet? For any upgrades to circuits to headquarters, data centers and other heavy-bandwidth sites, the business can switch to Ethernet access. TDM and Ethernet access circuits can be used simultaneously for WAN access, so there are no compatibility concerns. Branch sites can also move to Ethernet access in metropolitan areas, where the prices for Ethernet access have fallen rapidly.
I mentioned exceptions in my description of cap and grow. If you have branches with low-bandwidth needs, you may want to stick with TDM access, as the price-per-meg of Ethernet will likely exceed TDM for speeds under 6 Mbps. Don't forget to evaluate business cable service. Many of my clients are very satisfied with a business cable service. The handoff is Ethernet--just as it is in your home for cable Internet--so it can be very similar to Ethernet access from a provider such as the local exchange carrier.
My Law of Holes has many applications in IT--think IPv4 to IPv6, TDM telephony to VoIP, and traditional IT operation to cloud computing. If you recognize that a migration is needed, advocate for a technology migration. Show your organization why it's time to stop digging.