I upgraded my iCloud plan but I'm being charged for the old plan too

One would think so. But I would contact Apple iCloud+ Support for a definitive answer.

https://getsupport.apple.com/solutions

This would happen if both of them are going through different credit/debit cards. I've found that different payment methods sometimes confuses Apple Pay .But do contact the payment support team and they'll take care of this.

I pay $0.99 for the 50 Gig and I upgraded to 2 T...yet still being charged ..the $.99 now almost 1 year now...I tried contacting Apple Support...They just blew it off as its only 99 cents what the beef....