Why faces can change
Enhancement estimates missing detail. Dedicated face rebuilding can change facial features further, which is why it is off by default. Inspect eyes, lips, hairline and skin against the original.
Preparing an old photo
Scan flat and evenly lit. Avoid reflections, save a high-quality original, and repair damage before deciding whether to add color. Keep an untouched copy.
Choosing an enlargement
Start from the intended output dimensions, not a 4K label. Doubling width and height produces four times as many pixels. Review the output dimensions and credit quote before starting.