I need to go mumbai or Bangalore?

I’m planning to travel from NJP TO Mumbai OR Bangalore by train. I’ll be buying a general (unreserved) ticket planning to do it on general only im solo, but I’m considering boarding the sleeper class instead and then paying whatever fine or extra charge the TTE asks for. What are the actual consequences of doing this in Indian Railways? Is it just a fine plus the fare difference, or can there be other penalties/issues? Has anyone here done this before on a long route like this? Would appreciate any insight