Multi SRM Server version in N to 1 environment

Hi All,

I am going to upgrade N to 1 SRM 5.8 to new environment.

I have question on SRM server, is it must upgrade all SRM same time? Or SRM server can co-operation in different versions in same time?

DR Site: 6.5

Prod1 site: 6.5

Prod2 site: 5.8

Prod3 site: 5.8

Prod4 site: 5.8

I can find any KB or doc talking about that. Look any experiencer can share, thx.