One satoshi is a very small unit. If bitcoin is worth $100 million, 1 sat would be just $1.
Would such need to use smaller denomination units (smaller than satoshi) require a hard fork?
There is no hard fork needed, bitcoin can exist in milli, micro nona satoshi if satoshi has more value.
This is not requiring any hard fork at all.
Also worth to know that any other coin is altcoin, not bitcoin.