Fresh ink on an old tatt - rework

Hi guys

so I have this girl contacting me about putting some colour to her black tattoos, which are about 2 years old. I’d love to do it but the original lines are crooked and the whole design is a bit skewed so i thought I could correct here and there. so my question is - is „here and there“ even a viable option? Will it show if I just touch it up some places and not others, not knowing the brand of original ink? I understand a complete overhaul would be the way to go but is it necessary? Has anyone any experience with that sort of stuff?

Thanks in advance

I would only touch it up here and there if it were my own work I did it on. You don't even know if you use the same ink as the previous tattooer, all blacks don't look the same.

Thank you. Exactly what I wanted to know. I only use one brand and obviously have not enough xp.