What if I do not like the design?

You say so, and it changes. Nothing starts until the stencil is on your skin, you have looked at it in a mirror and you have said yes. Changing your mind at that point is normal and it costs you nothing.

The two points where you can change it

Both of these come before any needle, and both of them are yours.

When you first see the drawing · at the consultation or on the day, depending on your artist.
When the stencil is on your skin · this is the real one, because a design that works on paper can sit wrong on a shoulder.

How to say it

Plainly. "Can we make it bigger", "can it sit higher", "I am not sure about that part". No artist is offended by this and every artist would far rather hear it now.

If you cannot say what is wrong, say that instead. "Something is off and I cannot tell you what" is a useful sentence and your artist has heard it before.

What is reasonable and what is not

Adjusting size, placement, angle and detail on the day is normal and expected. Arriving and asking for a completely different subject is a new drawing, which means a new appointment · not because we are being difficult, but because the drawing takes as long as it takes.

If you have genuinely changed your mind about the whole idea, say so rather than going ahead. It is permanent. A rearranged appointment is a small problem and the alternative is not.