with enough players returning, i practice the new set an entire season before switching. at a new school, if you cut a ton, then you often want to switch mid way or so. but really its whenever the IQ gets close enough, you play the set that makes more sense.
slyman - i think you sort of made the point for the other side - the press IS the best defense to pull upsets, even in d1.
i guess i should qualify here - there is likely a difference in what you are talking about, and the OP - d1 and high d1 are VERY different animals, vastly more different than low/mid d1 and d2 or d3. the previous posters just mentioned d1, but georgetown is a high end school, probably trying to become an A range prestige club, and thus i am going to assume the implied context was high d1.
in lower d1, press can be VERY successful, i actually prefer press or zone to man at low d1, because all the top teams are playing d1 (well, thats an extremely simplified version, but you get the idea). its at high d1 where press gets a bad rap, and its for good reason. there are quite a few reasons, but some of the core ones are
1) volatility - this is a friend to lower teams, but enemy #1 to top teams - and the press is synonymous with volatility
2) early entries - they make it very hard to plan consistent depth across positions, which is vital to strong press teams. it also results in younger teams, which again, is not condcive to pressing.
3) recruit generation ratings - the ath and def ratings that make man defense tick are MUCH more prevalent in high end recruits than press driven speed
4) recruit generation disparity - the gap between the top d1 players and the "decent" d1 players is much bigger than it has ever been. at least for the last 6 years, until the new engine, it was never the case that the top players were so good, that there were always a group of maybe 15 elite d1 teams, with which virtually nobody could compete. the top teams are SO talented because the top players are SO talented, and then the dropoff is huge. so, top teams wont settle for less than spectacular recruits, and as a result, high d1 teams often take quite a few walkons. this makes it EXTREMELY difficult to run the press as effectively as man.
as you can see, these effects are much more present in high d1, than in low d1, if they are present in low d1 at all. by low d1 i really mean anything up to consistent NT team play, at least... anyway, because of this, press is much more viable for mid majors and low majors than for BCS schools. its actually a fine set for some BCS schools, lower end ones, if you play it right - but almost all BCS teams strive to get those top recruits, and man is really more suitable when that is the game you play. when i said 15 teams earlier, thats just in a single season - there might be 30 programs who can be one of those 15 teams any given season. so its really at the BCS level, not just A and A+ BCS schools, where press doesnt make a heck of a lot of sense. for lower BCS schools, where top players are less common and the dropoff between players smaller, and also, where volatility is more of a friend - you have something else working against you - with all that prestige other teams have on you, lower end BCS schools very often take multiple walkons every season, by design, to get some extra money, to help combat the major prestige disadvantages they have all over. press teams really need to be pushing to sign 11 or 12.