Yes, as always, call Equity, but...
I concur that it depends how long a time we are talking about.
I've subbed in on shows and the SM just paid me under the table. However, SHE was on a contract and I guess that's what I'd look for - that SOMEONE is on the contract. I would say that if it's more than a week then a new contract should be issued - thought process being that if a person is doing it for the week, they should get the points for health insurance. Without knowing all the details, I'd say the man replacing you should be on contract but the PM could sub in under the table. Grey area but that would be the battle I'd fight.
When is this all supposed to happen? Is it in two months, where there's time and they just haven't figured out the details and are still keeping dates fluid, hoping he can work out his conflict dates, etc? or is it next week and they're dragging their feet.