Couchbase Server Version Upgrade

Hi, We have a plan to upgrade Couchbase server community 6.0.0 to 7.1 community version.
The Couchbase Cluster has six Linux servers, and so does the XDCR server.
I have some question.
If we follow 6.0 → 6.5 → 7.1x step then don`t all Couchbase Servers need to be the same version at same time?
(some nodes 6.0 ver and some node 6.5 version in few min)
I read some comment in your document “To upgrade a Couchbase-Server cluster means to revise upwards the version of Couchbase Server that is running on every node.” it mean that Couchbase Cluster and XDCR server is not stop during Version upgrade??
We use Java SDK version 2.7.2 and 3.4.0. Java SDK version 2.7.2 is capability to Couchbase Server 7.1??

Since we are operating the service, please recommend if there is a safer way than the method in the link below.