Conservatism
Conservatism as a methodological principle says that we should prefer new theories that resemble our old theories. (I don't mean the principle that a new theory should be at least as good as its predecessors, nor the principle that it should explain the success and failures of its predecessors. Very non-conservative theories can do that.)
What is the status of conservatism? Is it a primitive rule telling us that even if we know that some revisionary theory is as good as a conservative one -- that both explain roughly the same data, make roughly the same predictions, are equally simple, etc. --, we should prefer the conservative theory? (An otherwise good theory according to which there are no birds, but only bird-halluzinations, say, just seems incredible, in particular if a more credible alternative is available.) In this case, conservatism would resemble the simplicity principle that tells us to always prefer the simpler of otherwise equal theories.
Or is conservatism just a practical rule, useful because it is very difficult to make up a completely new theory that works as well as our previous theories? This would mean that if we knew that a revisionary theory was as good as a conservative one, we would have no reason to prefer the one over the other. It's just that usually we don't know, and the odds are always against the revisionary theory.
Or is conservatism built into more central methodological principles? Take an empirical theory. Are the data it has to account for (explain, predict, etc.) just our sense perceptions, so that a brain-in-a-vat theory could account for them as well as an ordinary theory? Or do the data include all kinds of things we take ourselves to know: that we have hands, that there are birds, that fire causes smoke, that nothing goes faster than light, etc.? In this case, conservative theories are to be preferred because they better account for the data, not because they have the further advantage of being conservative. (For non-empirical theories, it seems even more obvious that the aim is not merely to account for sense perceptions.)