Is there a word for a change so small that it doesn’t seem to be a change at all?

A phrase I might use is a negligible change, which is often used in the Sciences to mean a change small enough to be ignored.

However, in the context of Science, a negligible change can be noticeable - it's just so small that it won't affect the system or experiment. Therefore, this might not be the word you're looking for.

English doesn't get much more precise than this...

Infinitesimal — immeasurably or incalculably minute.

It gets a bit mind-boggling when infinitesimal amounts are involved in, for example, homeopathic remedies that are so diluted there's only a very low probability of even a single molecule of the original substance being present. Which to me means an amount so small it's not actually there.

EDIT: I can't resist pointing out this answer itself now embodies an (almost) infinitesimal change. It was recently amended (not by me) to add a space before the first word. But you don't see it because of how the ELU display works (I only twigged by looking at the edit source).

I’m not aware of a single word that has that meaning, but you might try the following descriptions:

• a cosmetic change (a change that only affects appearance)
• a trivial change (a change that has no significant impact)

Imperceptible is very close although it can speak more to the perception of the viewer of the change more than the change itself.