There is an Ex light fitting certified with bulb A. It burns out, you need to exchange. At stock you only have type B bulb. It works well, light is there.
Are you an Ex manufacture now?
It is not according to original Ex certificate.
Is it a major change? Yes. So it is a new Ex product? Yes. Are you an Ex manufacturer now? Yes.
Do you make it happen via an Ex certified repair shop using option B described in iec 60079-19? Yes. Is it ok that way to use it in Ex environment? Yes.
Keep up good work!