A few things I've noted do change, but they may not mean anything to your game.
1. The players know and keep track of the bonuses and penalties to rolls, while the DM does not have to. This is different from their tracking what they *believe* these assignments are and the DM tracking the actual figures behind the screen. Honestly, this is by far the most important and game altering change.
2. The progression represented is a direct linear ascent with no increasing cost. Basically the PC's ability is the previous penalty/bonus number, while the d20 rolled number becomes the (variable) bonus. This is only half of what happened before. [e.g. 3+d20 now, or 10,000+d20 now, rather than a d20 representing the potential and a +3 moving the represented ability up that frame 3 spaces]. This matters when the frame is defining the scope of the game.
3. Related to number 2 there are no diminishing returns from higher or lower rolls. The AC:20, 20, 20, 20, 20, 21 explanation in 2e was fuzzy on this, but it was trying to keep +5 magical bonuses, the maximum for those games, from allowing PCs to achieve target numbers beyond what they could reach within the game's limits. The d20 roll, as well as many other rolls, was a linear distribution that had its numeric symbol results representing a curvilinear relationship which did have diminishing returns. A bell curve progression typically over a linear one. Keeping results on that bell curve meant further rolls could be allowed by creating further granularity within a result (normally at the ends: 1 & 20). A 2nd d20 roll could be allowed, if target numbers were above 20. So could a third or a fourth, etc. While this could always go on the odds declined to infinite smallness. [i.e. 20 = 5%, rolling 2 rolls of 20 = 1 in 400 or 0.25%, etc]. By allowing these diminishing returns (both beyond a 20 and within the 1-20 spread of results) creatures with abilities outside the human norm, the game's default, could gain logarithmically larger results with only an ability score of 19 or higher. (And very small creatures could do so as well when 2 or below.) This representation means numbers are smaller and easier to handle at the table. Results from a pure linear progression require several factors larger numerals to cover similar results. [e.g AD&D's STR 19 is d20's 25, STR 20 is d20's 32 or so. Though those numbers are blind guesswork].
