There is another letter-writing campaign underway to make it clear to WB Feature Films that not only will this film draw an audience with its original cast, that's likely the only way it will draw an audience.
I'm not usually a fan of letter-writing campaigns... everybody thinks they're John & Bjo Trimble now, everything is worth saving, and it's possible to save everything. They're usually wrong. (Example: Crusade was a lost cause from the beginning. People who wanted to save it were no longer in a position to pay for it. People who were in a position to save it were the people who killed it in the first place.) But this is different. Casting is not complete, filming has not started, this can be fixed now if enough people can politely point out the error of logic to the suits in question.