since+[past tense]

Dear teachers,

I'd like to use the following sentence in a past context:

# They have been successful ever since they opened for business.

I think I have to change have been to had been. But I don't know what I should do with opened. Can I change it to had opened? If no, why?

I will appreciate your help, thank you in advance.

[Tdol]

Why not use 'since opening'?