"We are not going back to a 1st or 2nd edition means of creating monsters. Those editions had no standards for monster design. Everyone just eyeballed it and hoped it was fair and fun (often it wasn't).
Third edition gives the illusion of fairness by giving you formulas to rely on, but you can use all the formulas perfectly and easily end up with an unfair or unfun monster. Advancing monsters by hit dice is a great example. Depending on its type and ability scores, the CR raise you give it according to the formulas might work out okay, but just as often the monster ends up too tough for its CR or too weak.
CR is often just a shot in the dark. We usually get it right, but I'm betting you can think of some critters that are way out of their weight class.
For each level of play we're devising a range of numbers for monsters that provide fairness and fun. Those numbers are based on what the PCs bring to the fight in terms of their potency and defenses, and upon the general role in the fight a monster is likely to be in.
Thus, the ogre, who is most likely to be the tough brute in melee, uses the “brute” range of numbers for its level. The numbers in that range and their distribution are designed to be fair and fun in a fight while at the same time allowing the artillery monster (like maybe a gnoll archer) of the same level to feel different but still be fair and fun. Of course, an ogre can chuck spears and that gnoll archer can charge up and hit you, but the numbers are devised in a fashion to produce great results when the monsters are used how people normally would use them. The ogre that’s in your face has more hit points than the gnoll archer that is using the ogre as a shield.
Changing a monster will be easier and more fair that ever. Rather than jumping through hoops and doing a lot of math with uncertain results, you can just look at the numbers for where you want to be and put the monster there. You might get there by adding a class, by "advancing" a monster, by adding a template, or some combination. The key is that you'll know where you need to get to in order to make the monster work right."