I currently have a basic $9.89 and Standard $14.89 edition of a program I sell. I will soon have the Professional edition done which will be subscription based at $29.89/year. My question is, how should I handle upgrades for existing users and should I require the one time purchase of the standard edition to use the pro edition?

Here's what I was thinking of doing:
Pro edition doesn't need purchase of standard edition, existing standard users get $10 off their first year of the pro edition, existing basic users get $5 off their first year of pro

Is that a good way of handling existing users and new users, would there be a better way?

Thanks