ISLing two switches with different VSAN ids

We are planning to merge two fabrics in our environment.

1. MDS9222i with a code level of 4.2 (1a). Has vsan 100

2. MDS9710 with code level of 6.2 (11b). Has vsan 2

Both have different VSAN numbers. So these are my questions

1. Do I need to upgrade MDS9222i to 6.2 before the merge? I know we need an upgrade badly, but should all switches in a fabric be at the same code level to merge?

Below are the steps that I know of , cause I have only merged new switches into an existing fabric before.

1. Check the priority of MDS9222i as I would need to keep this as the subsidiary switch. Change the priority to something lower than that of MDS9710. (Is this process disruptive?)

2. Create empty vsan 100 in  MDS 9710 and empty vsan 2 in MDS9222i

3. Create a port channel on each switch and trunk them.

My questions are which vsan should these trunking port channels belong to for the merge? Can they be in any vsan, either 100 or 2 or default. Or would I need to create separate trunking ports for ISL per vsan?

How can I ensure that the vsan merge occurs but one fabric doesn't wipe out the configuration of another.