The Zodiac Killer likely switched to murder by knife at Lake Berryessa because of the contast he noted in newspaper coverage between his crimes and the double murder of Kathy Snoozy and Deborah Furlong in San Jose on August 3rd 1969. The Zodiac Killer had murdered three people, attempted to murder a fourth, and mailed four letters and three cryptograms to high profile Bay Area newspapers. Imagine his "consternation", when discovering that the highest circulation newspapers in the Bay Area ignored his demands and relegated the cryptic murderer of three into relative anonymity on their inner pages.
The San Francisco Chronicle published his cryptogram on page four, while the San Francisco Examiner had placed their portion of the cipher on a lowly page nine. This was shortly followed by a comprehensive front page article in the San Francisco Chronicle on August 6th 1969, covering the brutal stabbings of Snoozy & Furlong in San Jose. The Zodiac Killer, likely scouring the newspapers for information about himself and the ongoing effort to crack his 408 cryptogram, must have read this article and been understandably horrified. He now knew what he had to do to secure the front page coverage he felt he richly deserved - kill his next victims with a knife. This was probably the trigger for his emphasis of "by knife" written on the car door of Bryan Hartnell on September 27th 1969.
If this was a copycat as police suspected, then we would have to conclude that the perpetrator (who didn't write the Belli letter) made the connection to the Frances Brown murder, which was concealed in the wording of the December 20th 1969 letter, and recreated her murder scene in the apartment of Robert Salem, with the Zodiac pseudonym attached. Or, the copycat inadvertently created the murder scene of Frances Brown in the 745 Stevenson Street, San Francisco apartment of Robert Salem, without any prior knowledge of the hidden meaning in the Melvin Belli letter, and just so happened to write the Zodiac pseudonym. The third option, is that the Zodiac Killer was responsible for the stabbing of Robert Salem and was deliberately linking the hidden message in the Melvin Belli letter to his latest murder, playing games with the police along the way. The final option may be that no connection exists between the wording on the Melvin Belli letter and the murder of Frances Brown. It is just another case of seeing things that aren't there.