It’s quite easy to tape the rims and swap the spokes across one by one. Contrary to what has been said above, if you deconstruct the wheel, you shouldn’t re-use the existing spokes – they have to stay in their existing configuration as they have been subject to long-term stress in a particular twisted position, so re-using them in a different position would mean re-twisting them and could lead to snapping.
As your spokes are 1.5-2.0mm too long, there could be a problem of strength. Again, contrary to what is said above, longer nipples would not help (they help only where your spokes are too short). Ideally you have 10mm of spoke thread engaged with 10mm of nipple thread, but if your spokes protrude by 2mm, you’re only going to have 8mm engaged and the join is going to be weaker. You might just get away with it, but I would guess that 80% is on the margin for how low you can safely go. I would take a look at the existing build – if the spokes are already protruding slightly, i.e., they are already slightly too long, then I wouldn’t go ahead with those spokes. If they look slightly too short now, then I think you could safely re-use them.