Maybe a little bit weird question, but can the amount of gas, when:
is used(for changing an existing variable), be lesser than 5000 gas? If no, are there discussions in community to reduce these costs?
Kind of. SSTORE has a few "gotchas" about its pricing, here they are:
There's also the net gas metering EIP that was originally in the Constantinople fork but was removed last month due to worries about changing invariants. Essentially, that EIP would drastically lower gas costs for multiple changes in one transaction, since multiple changes don't all need to be committed to the state.