Monsters exactly as MM? Not necessarily.
The MM says:
Modifying Creatures
Despite the Versatile collection of Monsters in this book, you might be at a loss when it comes to finding the perfect creature for part of an adventure. Feel free to tweak an existing creature to make it into something more useful for you, perhaps by borrowing a trait or two from a different monster or by using a variant or template, such as the ones in this book. Keep in mind that modifying a monster, including when you apply a template to it, might change its challenge rating.
Let us make the Orc as an example.
The MM version uses Greataxe and Javelin as weapons, both at +5 to hit (+2 from proficiency and +3 from Strength). You could say that a tribe of orcs from a certain region has the tradition of fighting larger monsters and hunt flying prey from afar and that tradition is reflected in their weapons of choice; Pike (Reach weapon) and Longbow (Ranged weapon with very long reach).
The first thing you should do is change their statistics accordingly:
Greataxe: Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit: 9 (1d12 + 3) slashing damage.
Javelin: Melee or Ranged Weapon Attack: +5 to hit, reach 5 ft. or range 30/120 ft., one target. Hit: 6 (1d6 + 3) piercing damage.
Change into:
Pike: Melee Weapon Attack: +5 to hit, reach 10 ft., one target. Hit: 8 (1d10 + 3) piercing damage.
Longbow: Ranged Weapon Attack: +3 to hit, range 150/600 ft., one target. Hit: 5 (1d8 + 1) piercing damage.
They kept their proficiency bonus and added the respective ability modifier to their attacks. They adapted to fighting Large creatures and adopted a reach weapon for safety reasons and while the hit chance from their ranged attack decreased, their range increased a lot, allowing them to attack earlier from farther away. These still are the same CR 1/2 orcs.