What work is needed in changing a two prong to a three prong outlet?
If you only have one or two places where you need three-prong receptacles, you don't have to figure out which is "first" (nearest the panel).  The NEC (since at least 1987) also allows you to replace ANY ordinary, non-grounding (two-prong) receptacle with a non-grounded, 3-prong GFCI receptacle. Marking it as having "no equipment ground" was added as a later requirement, as mentioned by Ecnerwal.