As long as the APICs are done first, you can do the remaining switches however you wish. We fully support running mixed versions wtihin a fabric as long as it's not for an extended period of time - I'd offer within a month at most. You'll understand why we'd prefer to limit running mixed versions below.
The most common & recommended approach for Switches are:
1. Odds & Evens Grouping. This requires 2 upgrade groups only as Spines & Leafs are done in tandem.
2. Odds & Evens Leafs, then Odds & Evens Spines. This requires 4 upgrade groups and doubles your upgrade/maintenance window.
3. Odds & Evens by Pod. Similar to Option 1, this requires 2 upgrade groups per Pod, and typically done across different weekends/maintenance windows to limit impact/risk.
Most customers go with the option 1 if they wish to knock out the entire upgrade in a single maintenance window, but I have seen customers do 3 also. They'll upgrade one pod, let it sit for the week or two, then do subsequent pods after. This ensures minimal disruption and allows them to gauge the operations before completing the remainder of the pods.
When new features are introduced (typically across major versions) they're not going to be functional on the switches until they've upgraded - so running a hybrid mix of upgraded & non-upgraded Leafs could lead to issues where new features are enabled from APIC, but only effective on a portion of the switches. You can see where this can cause issues.
Robert
