If people want to keep playing the D&D Next prerelease rules after the play test is over, that's a feather in WotC's cap. Since there'll be roughly 5-6 months between the last playtest download and sending the launch books to the printer, I'd expect to see quite a bit of change. But I very much doubt we'll be blindsided by a radically different product. Instead, I'd expect to see change in the 'player elements' (classes, races, feats), some adjustments to the subsystems, and a whole lot of additional material for spells, monsters and magic items - with the guts of the game (combat/skill resolution) remaining fairly consistent.