Joanne, Well said. This is many ways is the same point that I have been making for the past while.
Flexibility is the key if a new technology expects migration from an older one.
In general, my recommendation is to leave the interchange in tact so that other communication mechanisms are not affected. In the long run, the trading partner will have some type of directory with trading partner information in it. One of the fields pertains to communication mechanism preferred. Thus, the initiating TP can look up the receiving TP in the directory, determine what type of communication method is preferred and route accordingly. Flexibility needs to be left up to the TPs.