From: Seattle, WA, USA
I've been working with these cross-role upgrades and though this will all apparently become moot when the patch is released, I can relate some findings. First, any group that does not change roles is safe. A straight fighter group can upgrade to any fighter type, old and new, back and forth, without compromising any options. It's the groups that change roles that can break. As related, when an upgrade is made a slot is used. Example:
Fighter1 -> Fighter2 -> BomberA -> BomberB
In the upgrade pick list this group will show all fighter types plus BomberA. BomberB will not show on the list. If Fighter1 is upgraded to Fighter3, nothing bad will happen. The Fighter2 slot will be bumped, leaving the BomberA option still open. But if it at that point it is again upgraded to a fighter, say Fighter4 (or even Fighter2), BomberA will be bumped and the group can never upgrade to bombers.
There are a number of groups that take this form, with no intervening PDU off upgrade of the same type:
Fighter1 -> FighterbomberA -> FighterbomberB
If this group is upgraded to Fighter2, or Fighter7 or Fighter900, the upgrade to fighterbomber will be lost. Most of the Japanese Ki-27b to Ki-45 KAIa groups are like this.
There is another element to this. In the first example, when the fighter group is upgraded to BomberA all bomber types will now appear in the pick list and all fighter types will disappear. So when a carrier fighter group upgrades to a land-based fighter, like A6M2 -> A6M3, all land-based fighters, including N1K1-J, will become available. However, if the land-based fighter is recoverted to a carrier-based fighter (which can show up on the pick list if it was in the orignal PDU off path), the group reverts to carrier fighter and the ephemeral land-based option is lost.
Specific upgrade paths for individual groups can only be reliably determined by looking at them in the Scenario Editor (which is very easy). But you can at least tell if a group has a role change just by looking at its ingame upgrade pick list. If a group is able to upgrade to a different role, then one aircraft of the new role will appear on its pick list.