I agree with Elaine -- pretty much any other time of the year would be better than right now...
For a lilac, I really don't think you would hurt it, though, if you just cut the roots off, trimmed the top back a bit, and planted it. If it's quite rootbound, with the roots coiling around inside the pot, I would use a sharp knife to make a few top-to-bottom cuts through them (which will help new roots grow outward from the mass).