I remember a documentary, (late 70's?), that conducted an experiment along the lines of kempie's link.
The subject was fitted with glasses that inverted the image received by the eye - up was down and right left, and to look at your feet, you had to lift your head upwards.
Immediately after fitting, the subject was asked to pour "a cup of tea" - (a teapot of cold water and a cup) - and after waving the pot and cup about in the air and trying to move it certain directions very slowly, succeeded in making the table top very wet. Even walking was difficult.
After the week (or so?) of wearing the modifying spectacles, during which the subbject very soon adapted to walking, pouring cups of tea and many other tasks, the spectacles were removed.
At this point, being returned to 'normal vision' - the subject initially had the same difficulties she had experienced when first putting them on - ie. failing simple manual tasks, but very quickly adapted back to the previous "way of seeing the world".
The conclusion? Well, probably that there is no "correct way" of seeing the world (upside down or not) and that our brains very quickly adapt to interpreting visual input with our other senses - (for example, in the experiment, the subject quickly 'learnt' to turn her head right to see the source of a sound in her left ear)
The speed of reversion probaly had more to do with the subject's brain having 20-odd years of experience seeing things one way - and a little over a week being inverted, and therefore 'remembered' something of the old way of seeing.
So had the inversion be caused by some irreversible medical condition, it is probably reasonable to conclude that the brain would adapt quickly and easily.