The traditional way to handle this is by classifying a game according to the last cataloged position that occurs in it. This is how ECO classification works; yo

The traditional way to handle this is by classifying a game according to the last cataloged position that occurs in it. This is how ECO classification works; you can see its catalog of positions at http://www.chessgames.com/chessecohelp.html http://www.chessgames.com/chessecohelp.html. For example, just after White plays 1.Nf3, the game is classified as A04, but after 1...d5 2.d4, it's now officially a D02, over in the Queen's Pawn category, just as it would have been if the game had started 1.d4 d5 2.Nf3. Databases usually keep track of chess openings played by ECO code rather than by specific moves, exactly so that these transpositions are handled smoothly.