[cisco-voip] cucm, unity, uccx upgrade to version 7

[Sanjay]

i am trying to upgrade UC systems to version 7.
my company currently have CUCM version 6.1, Unity 5, and UCCX 5.

my question is this: my consultant company told us if we want to upgrade any
of the servers, i must upgrade all the servers(UCCM, Unity, and CRS)  to
version 7 at the same time??
is this true?  i hope they are wrong.

we want to upgrade CUCM first than upgrade the other servers later or
upgrade Unity and CRS first than upgrade CUCM later.

anyone done this upgrade these servers before?

thanks,
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