Political Shifts

The liberal policies of one generation can easily transition into the zeitgeist if enough people clamor for them or normalize the ideology connected with it.  Consequently, if they become part of the new status quo, later conservatives would then champion them because they have at last been made one of the social norms they are attempting to conserve or at best slowly phase out.  Conservatism, after all, is just about conserving traditions.  The exact traditions and norms the adherents are hoping to safeguard can dramatically differ across time and geography.

This is not to say that all ideas that happen to be associated with the conservatism of a particular time and place are false.  Reason is objectively true, and things like whatever scientific laws or moral obligations exist do not depend on subjective whim or the cultural spotlight.  Conservatism could align with these things on some matters and not on others, even if those issues vary over time.  Nothing is true or verifiable because it is philosophically customary in a given nation or community, though, so conservatives themselves have to be emotionalistic or nationalistic or fallaciously attached to social constructs and norms to even be conservative to start with.  

It is just that there is no single tenet intrinsic to conservatism other than that some sort of national/local tradition need to be conserved.  Whether what was once regarded as fringe liberal philosophy becomes mainstream because the older conservatives with power (who adjusted to inherited ideas and social structures they never rationalistically evaluated) literally die off or because of some other cultural phenomenon, specific ideas tied to liberalism can indeed become another generation's conservative notions.

Liberalism is simply about a push towards a future marked increasingly by progressive values (whatever that means at the time, not that it has to even be consistent with itself or other concepts being embraced).  Conservatism, on the contrary, is about resisting cultural changes or having them occur on a much more gradual basis.  For instance, a conservative opponent of the transatlantic slave trade would have held that, while this form of slavery should be abolished, it should be ended slowly so as to not upset an economy that rests upon it.

Political shifts still can happen, of course, and faster than a conservative might like.  The more probable pattern is simply that of one generation shifting cultural norms, for better or worse, and then becoming complacent or losing energy, perceiving the next generation that goes further than them (again, not that this makes their philosophy true or false simply because it seems novel or future-facing) to be too extreme and arbitrarily refusing to examine what is now quite familiar to the former.  Regardless, logical necessity and possibility are not dictated by popularity or timing in history; people who look to personal sensitivity or cultural trends instead of reason are delusional.