Presumably, if one were to simply move ETN from say Huobi to any new exchange that is based on the new blockchain, would the conversion between the two automatically take place during the sending/receiving process?
If you cannot answer this, (hopefully you can) then please can you push this question to ETN for an urgent answer?
When the time comes all exchanges will most likely have a period of being in maintenance mode at the same time. Imo I would of thought that this kimd of scenario wouldnt be possible, as they would all be offline to allow migration to occur at the same time.
If you were caught in that scenario, then the coins would just land in the wallet, that would close down and migrate when it was completed.
Coins are bridged over to the new blockchain after block 1811310, they wouldnt be able to create any transactions to send ETN somewhere as you suggested… when anyone logs into CLI (including exchanges!) it will create the bridging transaction and they then use their spend key to take control of the coins on the new blockchain.
Technically an exchange could not upgrade their node in order to stop this bridging, but they wouldnt be able to send coins anywhere until they do so wouldnt achieve anything.
Nobody should be leaving their ETN on exchanges, ESPECIALLY not during a huge bridging event like this.