IAP and existing users?

Hello all. I am looking at integrating IAP with my app but I am a little unclear on how I can do it fairly for my existing customers.

Currently, if you would like that app, you simply pay the fee of £5.99 and it's yours.

What I'd like to do is change it slightly so users who obtain my app in the future, can do so for 99p with limited features and then pay an additional £4.99 to upgrade it to the "pro" version if they like it.

The problem is, I don't want my current users to have to pay for anything else. Is there a way I can automatically grant them the "pro" version without having to pay again?