upgrading erlang servers

[Param Jyothi Reddy]

Hi guys,
I am new to erlang and was reading "Programming Erlang" by Joe
Armstrong and one remark in the book surprised me.

Pg 173: "And same version of erlang [on all nodes]. If you dont do
this, you'll get serious and mysterious errors.". Not sure how true
this is currently, however if it is, how do you upgrade cluster of
nodes running X version of erlang to Y without disrupting service? I
would have assumed that one can upgrade subset at a time which would
also provide a vehicle to test version Y on small subset before
committing the full cluster. But if this comment is true then one
would have to upgrade cluster of nodes in an all or nothing fashion.

There are other ways to upgrade by having two clusters and switching
load between them using some protocol. Is that how it is meant to be
used? Am i missing something obvious?

Thanks,
Param