Yes it would require a hardfork, and no one who actually understand the technology would support such a change.  And almost anyone in the bitcoin space would object to it, if for no more reason than it would increase the supply of bitcoin above 21 million.

This will never happen and there is no good reason for it to.  The risk of bugs alone is reason not to do it.  If you think "adding more decimal places" is a simple change you're mistaken.  For one thing, bitcoin has no decimal places.  In the code, there is no "bitcoin" it's only satoshis.  The 100 million satoshi as "one bitcoin" is completely arbitrary and does not exist in the code at all.

Bitcoin does not use decimal places, it used fixed point integral values.