How can you charge me interest?

I had an account with Edgars for years and it always was a 6-month interest-free account recently RCS took over the Edgars accounts after all the miss management. I just received my account from Edgars and apparently, they can charge me 18.5 present interest on my instalments now without me signing any documents or agreeing on anything LOL. Now the next step will probably be putting me on a Credit Bureau. I refuse to pay interest on an interest-free account.