Running old and new array in same group?

We have a ps4000 (sata, 16x1tb) that's running out of maintenance in may without ability to renew.

The ps4000 is a target for replication of windows/hyper-v volumes from a ps6100x and runs some hyper-v volumes with developer/test virtual machines.

We bought a new ps4210x to replace the old one. It would be run on 1gbit for at least the next year.

The best way to move the data is to update the firmware on the ps4000 to the same the 4210x has and then join the 4210x to the ps4000 group?

Is there a checklist what exactly is to do for moving the data from one member to the other?

We are unsure what to do with the old system. We thinking about buying one or two replacement disks in advance and using it for some more time. Using it as a replication target and hosting some not important virtual machines.

Would you suggest running both systems in the same group?

Should we use two pools for volume placement or would you just let the firmware load balance the data? Both systems have raid6.

What is with support, if we run both systems in the same group? Would we still get software support for the whole group or would Dell say one of the member does not have support anymore and we could not help you at all?

As I understand the firmware release notes, the 7.x will the last major version for ps5xxx. Our ps4000 from 2010 will have some more time with firmware updates?