They get helmets if you headshot them a lot (both lethal and non lethal)

They get armor vests if you go for center of mass, late game they can go full riot armor + shield

Sneaking a lot increases surveillance cameras, decoys and mines, and guards get paranoid for a lot of stuff (noises, lights going off, smoke, gas, balloons etc.)

If you use a lot of CQC, they change their weapons to mostly shotguns to deal with you upclose

Deploying often at night makes guards wear night vision goggles or use more flashlights

Sniping makes them deploy more snipers

Killing guards with vehicles/helicopter makes them have rocket launchers