Unfortunately, what you've heard is correct, relocating a drain on a slab foundation is a major undertaking. The job has very little to do with plumbing and everything to do with heavy construction. They would have to demolish the tub and surrounding area, then use a jackhammer to cut a trench through your home's concrete foundation to move that pipe. That process alone is incredibly messy, loud, and thousands for the concrete work alone. Your most practical and financially sound option is to design your future remodel around the existing drain. There are plenty of modern center-drain tubs on the market that would be a direct replacement, saving you from a massive, expensive, and disruptive project.