Will swapping the red wine in my spaghetti bolognese with port wreck the dish?
In short, using port as a substitute for red wine will not wreck the dish.

Though the flavour is different (and richer) and will make your bolognese taste different as a result, the taste should not be bad. I frequently do this as I am not a red wine drinker, and port keeps far better in an open bottle. I would recommend using slightly less than when using red wine, but this is highly subjective.