Right, just like how Bernie switched from targeting millionaires to billionaires once he became a billionaire himself.

I don't know anything much about the history of Sanders's rhetoric specifically, but: Inflation and economic growth mean that "millionaire" very much doesn't mean what it used to N years ago, and more so the larger N is. (And Sanders has been around for a while, so N can be pretty big if you're comparing early-Sanders with late-Sanders.)

If you believe something along the lines of "the richest 1% of society, the ones who have > 10x more wealth each than a typical upper-middle-class person, have too much money and too much power and we should change that" -- which I think is the kind of thing Sanders believes -- then talking about "millionaires" was a reasonable way to express that 50 years ago; these days what we need is a word whose meaning is more like "person with $20M or more"; give it another 50 years and "billionaires" might express roughly the same meaning that "millionaires" did in 1975. (Or, of course, there might be a huge economic crash, or a currency devaluation, or a technological singularity, or something.)

So someone could switch from complaining about "millionaires" to complaining about "billionaires" just because the way the meanings of those words have shifted means that the best word for pointing at a particular social issue used to be "millionaires" and is now "billionaires".

Because we really only have "millionaire" and "billionaire" and, more generally, numbers spaced by powers of 1000, the sets of people you can talk about pithily change over time. So, at the moment, you can talk about "millionaires" and be referencing something like the top 15% of US households (so if you're wanting to engage in some hostile rhetoric, pointing it at "millionaires" is probably broader than you want for several reasons); or you can talk about "billionaires" and be referencing something more like the top 0.0003% (so if you're wanting to raise money by redistribution, "billionaires" is probably much narrower than you want).

I suspect there are a few good PhD theses to be written investigating questions like "do populist-leftist movements have more success in places/times where some handy term like 'millionaire' picks out roughly the top 0.3%-3% of the population than in places where there's no word that does that?".

(Note: numbers above are in the right ballpark but I make no claim that careful calculation wouldn't change them somewhat.)

You probably meant to write "millionaire himself", and I don't know if it's true, but yes, same principle