Changing a bucket from port based authentication to SASL authentication can be achieved by changing the active bucket configuration. You must specify the existing configuration parameters and the changed authentication parameters in the request:

shell> curl -X POST -u admin:password -d ramQuotaMB=130 -d authType=sasl \
    -d saslPassword=letmein \
    http://localhost:8091/pools/default/buckets/acache