Now, sitting in the jardín — the loud, leafy central plaza — I began to deduce a complex answer.

A few weeks before my recent visit, San Miguel had been named a Unesco World Heritage Site, and at a nearby table, a group of Americans were buzzing about that success. Yet from the park bench where I sat, I could see something else that was new: on the facade of one of the carefully preserved old downtown buildings was the unmistakable logo of Starbucks.

This, in a nutshell, is San Miguel these days: balancing in a moment of almost exquisite equilibrium between new and old. But how long can it last? Can the Mexican residents, and the American expatriates who make up about 10 percent of the population, continue to have it both ways?

Talk to people around town, and you will find opinions mixed. For Jane Sallis, 65, who moved to San Miguel nearly six years ago after retiring from a technology business in Seattle, the current collision of 21st and 17th centuries is just one more turn in the path for a whimsical and eccentric town. “Change is inevitable,” she said. “Starbucks? The Mexicans here wanted it. It´s their town. San Miguel can handle it.” ...