To answer myself, I think the exchange event is this
"
The BBI EPS Issuer, at its election, may Redeem BBI EPS for cash or Convert BBI EPS to BBI
Stapled Securities or undertake a combination of those actions at certain times including:
...
at any time if the aggregate face value of BBI EPS on issue is less than $100 million; .."
According to my maths, this would be when BEPPA drops below 12.5c which it has indeed done.