it's possible to either switch between the (internal) circuits, or to enable just the input or enable just the output.
Philosophically, you could leave the input circuit connect at all times and simply ignore it (in software) when using the pin as an output.
Well, there is no reason to "disable" an input circuit and given the input impedance of CMOS, there really is no practical way to do so since the impedance of an open "switch" and an input buffer are much the same (or favour the input buffer).
The choice that may be made in the design of a microcontroller, is when an output is enabled, whether to (with additional complexity) read the state of the output register or the actual state of the pin itself which may (and particularly so with an open-collector driver) not actually be what is written to it.
This most certainly matters in respect of read-modify-write instructions whether implemented in the chip itself, or in code.