Break the concrete and do it right! Raising tubs, showers, and toilets to accomodate a drain change is never a good option. It will forever look like what it is...amateurish. It will also make the tub more difficult to get in and out of. Breaking concrete is not a super difficult job, there are rental tools that will make short work of it. Open the slab enough to get the drain exposed enough to work on, put the new drain in place and patch the concrete. The patch will be under the tub so it doesn't have to be perfectly finished. Just remember to wear eye and ear protection when breaking the concrete and keep the door closed so the dust is contained in the bathroom. To keep the hole a bit neater, score the area to be removed with a special blade in a circular saw before hitting it with the breaker.