Hi Susan,
These changes CAN come into effect pretty much from now. Your bank has to write to you (this could be a text, email, or alert on your banking app, instead of a letter) to let you know if your overdraft is changing.
Some have already introduced the new rates in July, including: Nationwide, TSB, First Direct, Barclays, and Santander. Halifax and Lloyds are already on 39.9% rates and didn’t change (unless customers asked). Banks changing in August include HSBC, RBS (Natwest Select) and M&S Bank.
If you have a £500 emergency overdraft, it’s important to find ways to pay it off ASAP as the 0% leniency that was introduced in April is ending – so you’ll start paying fees.