Rebalance exited with reason {not_all_nodes_are_ready_..}, after upgrade 1.8.0->2.0 when add 2.0 node to mix cluster

steps:
1. cluster 1.8.0r.deb 3 nodes( 1 standard, 1 sasl, 1 memcased bucket): 10.3.121.112, 10.3.121.113, 10.3.121.114
2. upgrade master node(10.3.121.112) with 2.0.0-1797-rel.rpm
observation: node 10.3.121.112 is pending state and working
3. add 1 node( 10.3.121.115) with version 1797 and rebalance

result: rebalance failed

2012-10-04 04:54:17.399 ns_orchestrator:4:info:message(ns_1@10.3.121.112) - Starting rebalance, KeepNodes = ['ns_1@10.3.121.112','ns_1@10.3.121.113',
'ns_1@10.3.121.114','ns_1@10.3.121.115'], EjectNodes = []

2012-10-04 04:54:17.431 ns_storage_conf:0:info:message(ns_1@10.3.121.112) - Deleting old data files of bucket "sasl-data"
2012-10-04 04:54:17.432 ns_storage_conf:0:info:message(ns_1@10.3.121.112) - Deleting old data files of bucket "1-data"
2012-10-04 04:54:17.438 ns_rebalancer:0:info:message(ns_1@10.3.121.112) - Started rebalancing bucket mem
2012-10-04 04:54:17.439 ns_rebalancer:0:info:message(ns_1@10.3.121.112) - Started rebalancing bucket sasl
2012-10-0404:55:17.452 ns_orchestrator:2:info:message(ns_1@10.3.121.112) - Rebalance exited with reason

{not_all_nodes_are_ready_yet, ['ns_1@10.3.121.113','ns_1@10.3.121.114']}

these nodes are still with version 1.8.0