There are two main issues with putting strings in the wrong position. Firstly the risk of damaging the harp, and secondly the likelihood of sounding bad, or really different from the rest of the harp.

Damaging the harp: You can put strings which are thinner than idea on with little risk as the tension on the instrument will be less. There is a slight issue that if you left several consecutive strings with really light tension compared to the others for a long time the soundboard might take on a slightly uneven shape. However, the only time I have seen an uneven soundboard the harp in question had been sitting with only a few strings on it for 50 years or so. Putting heavier strings on will increase the tension on the harp and whether the harp can take it depends on the harp, how much heavier, how many and for how long. Putting pedal gauge strings on a Grecian Erard instead of the right ones (which are about lever gauge) is the equivalent of moving all the strings about 5 places thicker and is known to eventually pull the harp to pieces. I am not sure about time scales: anecdotally some seem to survive for some years, others don’t. Some harps will withstand much higher increases in tension (I inflicted a major one on a Stoney End Eve and it seems to be surviving fine). Individual strings again depend on the harp, but the overall tension is less likely to cause problems – changing the lowest guts for wires is possible and unlikely to cause soundboard distortion as there is already a tension change between the gut and the wire.

Sound and feel: I notice a slight change when I put a string one out of step but soon adjust and am not sure I would notice if I didn’t know my own harp so well. I have also come across students unbothered by the fact that they have accidentally restrung an octave out.