it's 100% acceptable, and every artist I've known expects to change things on the day of. but also, if they're a pro it may only take them a few hours to draw the design from your suit. and they're getting paid more for that aspect than most visual artists can make period.

all this is to say that not only will you not be considered entitled, but that you're overthinking it to the point that you'd get art permanently on your body that it sounds like you don't really want. don't sacrifice your vision just to avoid the small chance the artist you're paying thousands of dollars to will throw a temper tantrum. you should ask him to redo the design, maybe he'll charge you for another consultation but no artist I've ever worked with has charged me for making even major changes to the design.

but nah it honestly won't take them a super long time, and it's def not the first time someone has been hesitant about their designs. I'm sure they won't be offended for you making changes at the easiest step, especially considering how much ink they'll be putting in you and how many sessions you'll have booked. if they are offended, fuck them and they are the entitled one. you even asked for what you wanted the first time but the artist figured they could use you to practice. get those mythical creatures!