Xendesktop 7.1 upgrade to Xendesktop 7.6

Here you go http://terenceluk.blogspot.sg/2014/10/upgrading-citrix-xendesktop-from.html
https://www.experts-exchange.com/articles/17059/Upgrade-from-Citrix-XenDesktop-XenApp-7-1-to-7-6.html

Couple of questions:  First let me explain what components we got here:

The whole infrastructure is on Vsphere 5.5

Citrix License Server 11.12.1 build 14008
Web interface Server 5.4
XD 7.1 with 2 DDC's
1 windows 7 master image , it is a VM with snapshots.
3 Machine Catologs.
Catalog 1: 2 Windows 2008R2 VM's with couple of applications. This catalog publishes those 2 apps.
Catalog2:  20 Vdi's using latest snapshot of master image.  This catalog is Non Persistent, randam pool and publishes windows 7 vm's.
Catalog3: 10 vdi's using one of the previous snapshot of the same master image. This catalog is same as above but just uses old snapshot of the image.

Now the order I will be doing is:

1)Upgrade 2 Windows 2008 R2 servers VDA version.
2) Upgrade VDA of Master Image windows 7 vm.  Take a snapshot , update Catalog2 machines with this snapshot ?
 Catalog3 will only be available temporarily as users are testing Catalog2 .  Once users are happy with Catalog2 then we will delete previous snapshots and stay just on latest one.

Bit confused on how I can upgrade vda for catalog3 ? Can vda 7.1 version co exist with 7.6 version ?
If so then I can leave catalog3 untouched and wait for users to come back and delete it.

Once vda's of images upgraded, then proceed with DDC 1 upgrade, then site upgrade and ddc2 upgrade.  Then Catalog and Delivery group upgrade.

Not sure if you understand my question clearly ? If not pl let me know. I can explain in detail.

Good Webinar.