Replace BUT you do know these cabinets don't match, right?

Replace and its okay if they are not quite the same style for a desk area and once they are all painted the same it will work.

I don't think you have given enough information to make the call, ie, whats the room used for? If its a kids room and you don't like the open shelf look simply fit a matching cupboard door and paint it all the same. If its an office I would replace the desk and overhead cupboards as they look cheap and nasty. Fit a nice desk and your new cabinet as it looks nice quality and would make a fine bookcase. If its in the kitchen/family I would still remove both the your old pieces for the above reasons, but if that top matches other counter tops in the room you could reuse it on better quality cabinet, Painted up your new cabinet would make a beautiful area to store your wine and whiskey glass's.

opps. just reread your post and missed the first two words, disregard the first two paragraphs