There was another handover at the beginning of this week - slightly less spectacular than that lachrymose display of imperial pomp in Hong Kong but no less fraught with anxiety for the locals. Indeed there are striking parallels between those Hong Kong businessmen who loudly proclaimed their optimism about "one country, two systems" in the run-up to unification, and the covert advertisements put out by some of Channel Four's senior editors - one close friend of Michael Jackson reports that the announcement of his appointment was followed by a sudden upsurge of "just between you and me" calls, in which various programme-makers shyly passed on their long-held admiration for the new chief executive. Large or small, all changes of regime probably generate much the same kind of survival strategies.

The big question, naturally, is what happens next. Some of the changes are likely to be evident rather quickly. Just as Hong Kong has already switched flags, I can't imagine it will be very long before Channel Four abandons the anaemic circles with which it replaced its lovable polychrome number. (Indeed the Channel might defray some of the reconversion costs by selling off the paperwork for the circles makeover as a perfect case study in how to bugger up your corporate profile.) But if the "4" is to be reinstated - and Four's image with it - then Jackson faces some more difficult tasks.

One of these is summed up by the ambiguity of the word "redefine", which will presumably have been uttered many times during interviews for Michael Grade's replacement. Does "redefine" mean a change in the definition or does it mean a reinforcement of it? Is the task to restore some clarity to an outline which has become frayed and fuzzy over the years or to adjust it for new conditions? When he was appointed, Jackson issued one of those ceremonial statements which are conventional at such times: "I greatly value the opportunity to lead an organisation which has transformed television," he said. "Channel Four has a unique programme proposition which I believe will continue to thrive in the future because of its special status which must be defended".

So, just as Hong Kong's "special way of life" is to enjoy protection for 50 years, Channel Four's "special status" is in safe hands. But the difficulty is that the first half of his statement explains why the second is considerably more wishful than true. Channel Four has transformed the television ecology and it has done so in ways which have steadily diminished its own protected territory. The improvements in BBC2, inaugurated by Alan Yentob and confirmed by Michael Jackson before he moved to BBC1, came partly in response to the new channel; they have made it increasingly difficult to say with any clarity exactly what is special about Channel Four. Add to this the paradox that Michael Jackson will effectively be competing with himself for a time (the inertia in television means that BBC2 is still in large part the channel he made it); to pursue the Hong Kong analogy a bit further, this looks, initially at least, as if it will be "two countries, one system".

Of course, Channel Four cannot be run as if it is a privatised BBC2. For one thing, it doesn't stand in relation to ITV as BBC2 does to its more popular counterpart. For another, its chief executive cannot simply order into existence the programmes he wants - the channel must coax those programmes out of independent producers who may not be equipped to deliver it. Such cultural adjustments, though, are likely to be less important than establishing a new sense of purpose for the Channel. Those who work there need to be able to answer the simple question "What are we for?" (Never mind that hardly anyone at the BBC could currently answer the same question - that's their problem.)

The conventional answer to this used to be that it was there for minorities not otherwise served by the terrestrial channels, but this has not exactly proved a reliable mission statement; it has been more useful as a defence for the mediocre (The Girlie Show, et al.) than as a generator of genuinely innovative television. Niches are gaps in which you can get your fingers badly trapped and for some time Channel Four's best programmes have also been their broadest in demographic reach. Besides, BBC2 is just as capable of serving audiences defined by exclusion as Channel Four is - 15 years ago, Gaytime TV would probably only have found a home on the latter but it passes without comment on BBC2 these days.

My own view, after a year in which it's arguable that all four main channels have slipped a little downmarket (that terrestrial cable station, Channel Five, can hardly be included in the reckoning) is that there is only one minority audience which it is worth bearing in mind for privileged treatment - and it is a minority that excludes no one on grounds of gender, race or age. The minority I have in mind are sceptical, intelligent viewers who seek a channel of resistance - resistance to the PR machine, to millennial delusions, to the diminished ambitions of so many television forms. What is Channel Four for? It is there, paradoxically, to improve its rivals - either by shaming the unimprovable out of existence (as Brass Eye tried to do) or by seducing the others into imitation (and not moaning about it when it happens). The way in which Michael Jackson sets out to achieve this will definitely be worth watching, even if not all of the programmes are.