In some cases, bands started out as "prog" from the get-go and generally remained in that genre. For example, I would say, King Crimson, Moody Blues, ELP, Flower Kings, Spock's Bear and even Porcupine Tree and Marillion all started as as "prog bands" (however you want to define the genre) and remained so through their tenure. Others had prog or psych tendencies and then morphed into prog bands, such as Yes and Floyd.
Others started out as hard rock or blues based and then also morphed (most temporarily in the early 70's since prog was the soup de hour), such as Rush, Tull and Led Zep.
Finanally, we have some BeeGee wannabes who became PROG like Genesis.
In terms of bands that make the great prog leap forward, I have to say Genesis going from Genesis to Revelation to Trespass was a pretty huge step, but what really blows my mind is Tull's change from Aqualung to TAAB. Every time TAAB comes on AOL's prog station I just can't get over how freakin awesome a song TAAB is, how "prog" it is and what a leap forward it is in terms of songwriting, arrangement, playing, chops and creativity over Aqualung (not that Aqualung is not a awesome album). I always think how did they do it! John Evans and Barry B really shine on TTAB.
What sayeth you?
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