It cannot be done easily. It can be done, if the majority of Bitcoin nodes agree with a given replacement solution.
It probably won't need to be done unless the human population increases by orders of magnitude and/or all live American lifestyles AND switch to Bitcoin as the sole global currency. So, probably not a problem.
Consider this: total global currency of all kinds is worth approximately $4.5 trillion. If Bitcoin were used in place of 100% of that, a Satoshi would be worth $0.002. That is, a fifth of a penny. And even pennies are basically worthless today. Since Bitcoin is deflationary/stable, a Satoshi will continue to be a certain percentage of the world's currency. I suppose the problem might be if the human population increases its real product by orders of magnitude, so that a Satoshi would have the buying power of $0.20 or even $2.00. At that point, I suppose people might be annoyed at not being able to sell something for less than a Satoshi.
In that absurdly far-future and unlikely scenario, the Bitcoin protocol could be replaced with something more precise. Or people could supplement their Bitcoin use with Litecoin use (or another cryptocurrency). Bitcoin could end up being the currency of choice for large transactions and Litecoin (etc.) for buying Pan-galactic Gargle Blasters and Soylent Green. Or banks could issue IOU notes for fractions of Satoshis. Or, you know, maybe people will pay with credit cards and the fractional reserve banking system and use fractions of Satoshis that are only recognized by the banking network. Or maybe we'll all be communist Star Trek people with holodecks and replicators who trick Ferengi into accepting Bitcoin.