Adjusting Finance (PCP)

Probably more UK based question - Has anyone gone through the process of adjusting their annual milage whilst on a PCP deal? I'm coming up to 2 years of my 3 year PCP and I'm way over already. I want to adjust and add another 2,000 on per year (retrospectively for the first year too) but wondered if it's going to be a massive hike in monthly payments?

For reference it's from 6,000 to 8,000. I have a meeting at the Kia Holden branch next week to discuss.