Quick silly question about tubes - K4040 amp


First, I love my K4040 amp…

I want to know if its possible to put a 12AX7 (5751) in place of the 12AU7?

Would there be issues with too much gain ? Will it harm the unit ?

I am unclear exactly what the 12AU7 serves as, can you clarify what it does, its purpose?

Thank you !!!

Sorry, we have no experience with this modification.
The 12AU7 serves as pre-amp in this circuit.