I'm still waiting for my electrician to replace my panel. But while waiting, I've been replacing 2 prong receptacles with grounded receptacles. I started with both regular and GFCI in hand so that I could install GFCI if there was no ground, but so far have found all the old boxes had ground available either as part of NM cable or armored cable. (Also fixed hot neutral reverse and other problems as I've found them.) GFCI is a great fix if you really don't have ground, but otherwise normally only required for new receptacles.
That being said, if you are replacing any receptacles in prime GFCI territory such as kitchen, bathroom or outdoos, upgrading to a GFCI is a really good idea. My electrician did that in my bathrooms years ago when I had him in to do other work - I don't think he even asked, just said "you need this, I'm doing it" and I trusted his judgment. Since then I found out about some GFCI trips from my family (they were real - bad nightlight) and am glad he installed them.