I suppose you could dowel and re-drill but unless you use a drill press or jig chances are even greater for off center holes. The bit will wander due to multi-directional wood grain. Can the leggs be rotated 90 deg for drilling new holes? Have you tried shims between leg and apron or mounting plate?

OR, if the top is thick enough (you don't specify, so this may be a reach) you could use dowel screws (regional alert: has lag screw threads at both pointy ends) started in the top first. That would be perhaps easier to drill a plumb hole into. Then each leg, if the end were truly square, would tend to self straighten as it was turned tight to the tops undersurface.

All sorts of possibilities. As for splitting the leg, you can indeed drill out the "old" hole in the leg and then plug it with a hardwood dowel and almost any sort of glue. You'll find, maybe, the end grain of a dowel easier to drill into precisely than the leg end grain. Start with a nice 1/8 inch bit and then widen that pilot hole appropriately.

At a place called The Door Store in Washington we made scads of tables both ways. The lighter and thinner tops, such as melamine over particle board, did well with the plates and the heaver and thicker tops, such as butcher board, worked better directly wood-to-wood. The metal plates made it much harder to do a decent skirt, too.

Thanks so much!

I'll try a shim first, and see if that works, and a dowel pin if not.

The table will have a fabric skirt when it's finished, so a tiny bit of crooked doesn't matter as far as appearances go, but I'm more concerned about it being structurally sound and not wobbly.