Once upon a lifetime is a phrase that can be alternatively substituted by a range of synonyms that convey the same meaning. Some of these synonyms include, 'once in a blue moon,' 'once in a lifetime,' 'once in an age,' and 'once in a while.' These phrases all suggest rarity and something that is not likely to occur very often. They convey a sense of something precious and unique, be it an opportunity, an experience, or a moment. All of these phrases are often used in storytelling, where they evoke a sense of wonder and enchantment, wistfulness, or nostalgia. Ultimately, whether it's 'once in a lifetime' or 'once upon a lifetime,' the message is the same - cherish the moment and make it count.