I called to modify our park tickets to remove the park hopper for our upcoming October trip. It was an experience! In addition to being on hold for three hours. They can’t just refund you the price difference, because of the park reservation system, they actually had to repurchase all new tickets for my party, link those to my MDE account and then cancel the tickets with the park hoppers. If it doesn’t work correctly there’s a chance that you’ll lose not only your park reservations but also any dining reservations you have linked to those tickets. I also bought my tickets in February before the price increase. I didn’t realize that they didn’t give me my original price, so I’m a few hundred dollars short of the refund I should have been given. I really need to call them back about that, but it’s hard to dedicate another 3 or more hours on hold for that.
I purchased our tickets directly from Disney so I can’t help with the second part of your question, but my guess would be you would need to call where you purchased them originally.