That's interesting. And I certainly don't discount that's what Dan Curtis said. But back in the '80s, when Sam Hall was interviewed on the subject, according to a published transcript the exchange went like this:
"Q: Did you ever give thought to what the Jonathan Frid sequel film would deal with?
A: We had the whole plot worked out for Jonathan when he said he wouldn't do it. I don't think he ever read it."
And that conflicts with DC's version. Though I can't say that surprises me all that much because through the years DC had often gotten his facts wrong. And one need not simply look to his latter years for evidence of that because even while the original DS was still on the air there were interviews where DC got his facts wrong. The most infamous was probably the one in which DC was certain that Sam Hall had begun writing for DS before Grayson Hall began working on the show and that it was through Sam's association with the show that Grayson got the job as Julia. However, we all know that Grayson began working on DS in June of '67 and Sam didn't begin writing for DS until November 0f '67 - and it was actually Grayson and Sam's socializing with DC after she'd begun working on the show that got Sam the job.
As far as discussing the matter with Sam Hall goes, I wouldn't doubt that his memory of things might be a bit faulty nowadays. That happens to nearly everyone at his age. But I'm more inclined to believe the things he said back in the '80s when there wasn't as much of a time distance and things could have been much more clear in his mind.
Though, honestly, who knows what the truth really is. I seriously doubt we'll ever really know...