In the interview audiotape with Grayson Hall from March 1973, she clarified that it wasn't that Frid did not want to play Barnabas anymore, it was that he did not want to be a vampire and have to do "the fanging" onscreen again.
Although I do not know of any published accounts of this, I've seen references in other posts over the years to Frid walking off the set at some point in the Fall of '70 and not coming back until he got certain things in writing from DCP. I've always thought that one of his demands must have been that he get to play another character besides Barnabas. I would guess that the germ of the 1841 PT story sprung from this; Curtis may also have been making plans for several of the cast members having to be away for several weeks during the filming of the second movie. I remember seeing a clipping from a magazine published in Jan. of 1971 introducing Keith Prentice to teen readers and commenting that he had signed a "five year contract" for DS. Difficult though it seems to believe now, the DS TPTB may have been hoping to have Prentice be the hot new male property on the show.
I watched right up till the end and I recall tuning in every day thinking they would go back to Barnabas and Julia in 1971. I think a huge number of viewers must have had similar expectations which accounts for why TV Guide published Sam Hall's article in the Fall of '71 giving Sam's version of the fates of the present day characters on the show. At the time, this was a VERY unusual thing to turn up in the magazine. I still can't quite recall anything similar. Maybe fannish outrage over the denouement of the BEAUTY AND THE BEAST series in the 1980s was comparable--that's the only thing that comes to mind; that and, of course, the cancellation of STAR TREK at the end of the Sixties. But still, TV Guide never ran an article on "here's what happened to Capt. Kirk, Mr. Spock, and Lieut. Uhura after the show ended."
G.