If you want 50/34 chainrings, you have to replace your whole crankset.

Think of it this way. A chainring with fewer teeth will also have a smaller diameter. At some point that diameter becomes less than the crankset's bolt circle diameter.

Typical road bike double cranksets have a 130mm bolt circle diameter. I'm told that 38 teeth is the smallest chainring that will fit.

Compact road cranksets have a 110mm bolt circle diameter so smaller chainrings will fit.