Still working as of two weeks ago but IME really depends on how you approach this.
Best result I've found is to play dumb and ask to drop the segments. If they come back with change fee, act surprised and HUCUA.
On the other hand, I did get a couple of agents who insist on charging a fee when switching carriers while keeping O/D the same.
Originally booked:
FRA-MAD-MIA-JFK on IB/AA
Switched to (no fee)
FRA-PHL-BWI-JFK on US/AA
Afterwards, several agent attempted to charge fee when switching to
FRA-TXL-DUS-JFK on AB
Had to HUCUA