Pretty unlikely. It was miraculous enough for the Democrats to make the PV gain in 1970 that they made, but to actually defeat Reagan? That would be difficult.
However, that doesn't mean it's
impossible. As with anything in history, as my theory goes, this can always be changed. Let us assume that our POD is Governor Reagan being less successful a Governor; he is Governor in 1969, a very crazy year: Therefore, for the sake of argument, let's assume there's a hippie protest in California in 1969 that Governor Reagan screws up for whatever reason it might be -- being misinformed, orders failed to be carried out, etc. -- and Governor Reagan is seen as a failure. Reagan is excellent at public relations, however, so it would take a few incidents like these for them to really take its toll on the Reagan administration.
Therefore, let's assume Reagan's luck turns out be HORRID. By 1970, many more Californians see Reagan as a screw-up, and so, by a very, VERY narrow margin, Jesse Unruh defeats Governor Reagan in November 1970.
Of course, there is a plethora of PODs this unleashes. The biggest we can see is, obviously, that Ronald Reagan never becomes President. After his 1970 defeat, I'd be willing to bet Reagan goes back into acting, and although he will be mentioned as a very famous/infamous California Governor and will be remembered, he does not have a future in politics anymore. He doesn't challenge President Ford in '76, and I would be willing to assume this leads to Ford's narrow victory over Jimmy Carter. I believe this because Reagan's primary challenge weakened the Republican base drastically -- conservative voters stayed home, and even your generic Republican voter had lost faith in Gerald Ford's ability to keep the nation unified ("If he can't keep his party together, how can he keep the
country together?"), and the promise of unity was most important in post-Watergate America and the 1976 election. So with Ford keeping the party together, he beats Carter/Mondale.
However, let's skip back a bit. Jesse Unruh is Governor of California, starting January 1971. "Big Daddy" was a moderate/conservative Democrat, a fervent and leading supporter of Robert Kennedy in 1968. Unless California is more chaotic as a result of our POD, which I could see occurring, I see Governor Unruh as a more successful Governor. However, I don't know enough about Unruh to be 100% sure.
What I am sure of is, from what I know, Unruh would become a leader of conservative Democrats nationwide. If he's re-elected in 1974, I could see him becoming a presidential candidate in 1976. However, I'm not entirely sure if he can win -- it depends on whether or not Scoop Jackson still enters the race, and how well he can fight with Jimmy Carter and other Democratic candidates. It may be a close race, however, and although I still believe Carter would win out, Unruh might even win second place, and once Carter loses, is a frontrunner for 1980.
Once 1980 comes around, Governor Unruh is the frontrunner for the Democratic nomination. The Republicans have been in power for 11 years, and that means voter fatigue for the GOP -- and that means, very likely, that whoever the Democrats nominate, will become the next President.
Unless an insurgent can defeat Unruh, I see Unruh becoming the Democratic nominee, and defeating Vice President Dole in November 1980 by a large margin.
After that point, I really should stop talking, because I really don't know much about Jesse Unruh enough to talk about President Unruh.

I'll let someone else talk about that.