I was going to write him privately, but the response deserves a public thank you. The question is complicated not only by the structure of the spectrum management process, but also by the tremendous variety of radio based services. The move toward greater flexibility is evident over time, such as when comparing the greater flexibility the FCC granted PCS licensees to that originally granted to cellular licensees. There are and will continue to be variations, so that it is difficult to draw conclusions from, say, the flexibility granted to Wireless Communications Service licensees as opposed to the use restrictions on broadcasters.
Observers categorize flexibility into several types.
This paper provides a useful overview issues of market based spectrum management. In that vein, one quibble I have with Charles Iseman is his identification of auctions among the principles used by the FCC to maximize licensee flexibility. Licenses awarded by auction, however, grant the user no greater flexibility, in fact no greater rights at all, than licenses awarded by any other assignment mechanism. Auctions are a faster, more efficient, and arguably more fair way to award licenses in many instances, but they do not by themselves add flexibility to spectrum use.