Because of this change, the representation of a vclock through the PB interface changes from a binary to a base64 encoded binary (see riak_object:decode_vclock).  So, if a node is upgraded in a cluster, vclocks fetched from other nodes will be incompatible with this node. For instances where the vclock is sent to a different node than the original one, I believe this will fail.  This is in addition to requests failing when getting a vclock from a cluster right before the capabilities change to a new format, which we should probably handle too. Thoughts?