The rear main is a $10 part and the only special tool you need is a torque wrench (fly and clutch), which is cheap and a good item to have anyway. I just had to replace mine. Unless there is damage to the block in that area its a cheap fix...monitarily...and then the motor is fine (assuming all else is well). So unless there is some other reason to swap the engine you don't need to. I would be worried about what it is being replaced with. The guy doing it probably knows that its an easy and inexpensive fix when the engine is already out. Is he replacing it with some POS junk yard motor that never had an oil change?

And are you sure it is the rear main? Could it be the oil pan? Twice as costly ($20) but much easier to get to.