For new additional wheelsets you can go as fancy as you want and just use the spacers as already noted.

For the current wheelset there's absolutely no big disadvantage to lacing on a new rim with new spokes other than adding a bunch to the cost of rebuilding the wheels. To keep it simple stick with rims that have the same ERD as the present ones so you don't need to replace the spokes. This assumes that the nipples unscrew easily without any undue excessive spoke twist. Sometimes corrosion in the threads can render the old spokes useless as too much damage is done taking the wheel apart in the first place. But if you can unscrew the existing nipples pretty easily then I'd just tape the new rims to the old, slacken off all the spokes and then one at a time remove the nipples and move the spokes to the hole in the new rim. But before I did this I'd suggest you clean up and relube the hub and check on the freehub bearings. If there's much of anything wrong with the hubs in terms of ball race wear or corrosion or the freehub isn't smooth running then it may be time to just retire the wheelset. But if it runs well then the cheaper and highly effective plan of just lacing on new rims is the way to go. Keep these as your big tire wheels and then use the deeper V section minimal spoke wheels you want to get as your quick road set. Overall it'll end up a lot less costly that way.