Goto Bathroom Pictures
Since the old shower was really a bathtub with shower combination, the drain was located at the end of the tub where you would normally find the drain in a bathtub. In a shower, on the other hand, it is customary to have the drain centered on the floor of the shower, with the tiles forming a sloped surface to guide the water to the drain. This means that the drain for the bathtub to shower conversion would have to move several feet from its present, original location.
Fortunately this house was built with a crawl space meaning that there's room under the floor for access to of all the plumbing for the shower drain. The workers will need to cut out some of the floor board to get enough access to the crawl space to remove the old shower drain pieces and re install new ones in the correct location.
If this house were built on a concrete slab, the some excavation would be required. Concrete would need to be cut out from the present drain location to the new location, exposing the soil underneath. Since the drain P-trap would be under ground as well, the soil would have to be dug up, and enough removed to gain the working space needed to re arrange the plumbing location. This would no doubt take time, and cause undue noise and mess. You might not be so lucky - your house might be built on a slab.
I'm not sure why bath tubs always have the drain at one end of the tub, but I can guess that it makes it easy to install the tub so that the water collects on one side of the tub - in this case, the side of the bathtub with the drain. On other words, simple door shims could be used to raise one end of the tub to cause all the water to head for the other end, and down the drain. Of corse, my theory could be all wet, pardon the pun, but it seems to make sense.
One other last thing worth mentioning now that the project is complete - the screen over the drain uses large 1/4 inch diameter holes which allows large droplets of water to drain out. That can get kind of noisy to the point of undue notice compared to the elegance of the new shower. Fortunately, these screens come in standard sizes, and ones with different drain holes are readily available, and much quieter.