Oh yes. The popcorn line is only one of several that she used in the 1970s. Most of them did not survive beyond that period.
In the beginning, Peggy ended just about each verse with a different humorous or sarcastic line. The most daring of those lines was the one that followed the verse "and then one day I fell in love with the most wonderful one in the world." Her addition:
And then he died on me.
That is a very loaded addition, of course -- and one that carries an autobiographical undertone, too. But it was all in the delivery: she usually uttered it in such a tongue-in-cheek manner that concert audiences would burst into laughter. Ditto for her other additions to the song. Such humorous performances of the number invariably ended with loud, excited applause, and plenty of laughter.
However, that reaction was not unanimous. Certain isolated members in the audience were not happy with the injection of humor: they had come to hear the more serious, dramatic interpretation of the recording, and they would express her disappointment to her when they visited backstage. That was probably one of the main reasons why she refrained from continuing this fully humorous approach as the years went by. (She still tried it from time to time, but in a more limited capacity. The line "he died on me" fully disappeared. The popcorn line was the one kept the longest, being the lightest one, and the one that was connected to the least tragic of the verses.)
As for the line "we would take long walks by the river or just sit for hours gazing into each other's eyes," it is my belief that a personal sense of logic let her to drop the words "into each other's eyes." Mind you, there is no problem with these words if you take them figuratively. But, if you take the words literally, the notion that any two people would spend hours gazing into each other's eyes might strike you as risible or, worse, ridiculous.
As you said, some worthwhile, long-established singers feel a need to freshen up numbers that they keep on performing over the decades. In this particular case, there was also Peggy's repeated claim that she couldn't really sing something that she didn't feel, or which she didn't relate to. (Obviously, she could have, at least in theory. But her heart would have not been in it; that's what I think she meant.) I imagine that there were times when she "wasn't feeling" the heaviness of "Is That All There Is," and thus wanted to go for a lighter approach. Especially in the years right after the number became a hit. The song was always a challenge for her, because numerous listeners thought of it as a nihilistic number, while she strove to put a relatively positive spin on it.
During a 1970s press interview, Peggy declared that, as long as audiences kept asking for them, she was happy to continue to sing her hits. That's one of various comments of hers which suggest that her singing of old, past hits was done as a concession to audiences -- i.e., out of love for them, rather than due to any strictly personal interest, or desire to rest in her laurels. This artist's excitement was reserved instead for numbers that were new or fresh to her.
"Fever" also underwent humorous treatment over the years. The cartoonish voice on "what a lovely way to burn" is just the tip of the iceberg. If you listen to her rehearsal of the song on the DVD
Fever: The Music of Peggy Lee, you will hear the chicken line, which she seems to have also used in actual concert. Then there was her "magic act," which seems to have been added in her later years.
I believe that humor, as already intimated by Richard, was an integral part of Peggy's life. (The recent biography's virtual erasure of this dimension of her personality is, for me, one of its several major shortcomings. It serves as an indicator of how unbalanced a portrait it is.)
Ivan