• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Can I replace 6H9C (6SL7) with 6SN7?

I am unsure which direction you are going, as the thread title and your last sentence seem to say opposite things.

Obviously you need to be able to supply the required heater current.

It is unlikely that a well-designed circuit for one of them will also be a well-designed circuit for the other, as the two valves have different characteristics. However, if you are not fussy about sound quality or the circuit is badly designed for the valve it has then a swap may be worth trying.