Is it safe to replace a GFCI breaker with an arc fault breaker?
GFCI are still needed because ACFI do not provide the same level of ground-fault protection as GFCI. In the United States, the GFCI is triggered at 4-6 mA of current-imbalance. The trip-limit for AFCI devices is a current-imbalance of 30-50 mA. So AFCI devices do not provide the same level of ground-fault protection as GFCI devices. You may want to use both an AFCI and a GFCI on your circuit.

In addition to ground faults, AFCI devices do also detect arcs. GFCI and AFCI are designed to detect different faults from eachother.

GFCIs are still needed near sources of water (like the kitchen and bathroom), in unfinished basements, and outdoors (among other locations) (and even if the circuit uses an AFCI breaker).