The primary and alternate databases should be equivalent in terms of rule sizes, and mirror images of each other to help prevent the loss of data. If you wish to switch to non-equivalent databases, you must stop and restart CA ACF2. Since the ALT databases are 4k, and the new PRIMARY databases are 16k, the SWITCH command fails.
S ACF2 <== the default START is the primary databases, so that will resolve the problem. ACF2 can be started pointing to other databases defined in the ACFFDR.