> Matter/thread is nothing like zigbee, except the MAC layer is the same.

Matter actually uses an iteration of the Zigbee device model - from an application level they are actually somewhat similar

> I don't know why you think your zigbee devices will be upgradeable to thread - that strikes me as quite unlikely

Most Zigbee radios are dual Zigbee/Thread since both protocols are build on 802.15.4. I suspect that the limitation is going to be on manufacturers rather than hardware limitations