@nescafegirl89
Let's say that customers who don't prepay get charged $100 while customers who prepay only get charged $90. When the prepay discount was removed, do all customers now have to pay $100?
The company probably makes less money after the change because the "discount" has always been artificial and their price is subject to competition. Let's remember that technically both people in your example should be billed $90, and if late an extra $10. Whereas before both were billed $100 inclusive of the pre-applied late fee that the power company remove later, which makes it look like a discount. Now the power company can't pre-ransom the fee so they can't invest it, and it probably helps people avoid the fee if they have to be proven to be late, rather than having to prove that they are not late. The burden of proof is now on the power company.
It also reduces obfuscation which leads to more healthy competition, and reduces hardship for those whom can barely afford it to begin with. Overall I'd say this reduces profit for the power companies, allows people to keep more of their own money and makes the system fairer.
However, the caveat here is that it's dependent on the company and whether competition and if the free market decides to adjust the price. For example, if there's only 1 power company or if all the power companies colluded, they can just keep charging everyone $100 with no offer of discount and justify it because "the government removed the Prompt Payment Discounts (PPDs)" and just keep the price as-is while removing the "discount"; effectively increasing the price. In such circumstances, it will lead to higher profits because they didn't need to compensate alongside everyone else.
So, it's not necessarily the case that power companies will profit more with the change, but it is obviously theoretically possible. In reality I'd think that they'd profit less, but I'm not an expert economist or energy specialist. However, let me leave you with a question that may help us answer at an intuition level:
Do we really think the power company bean counters would offer PPDs in the first place if it wasn't more profitable/advantageous than not having them?