Political parties in the United States have platforms under which they gather around. These can be broad or specific in scope and they can change over time. My question is whether there is a governing body that is responsible for governance of these changes? Is it different for the two major parties versus the smaller parties?
The platform of a political party is set at its national convention every four years in synch with the Presidential election for both the Democratic Party and the Republican Party in the United States.
All major political parties in the United States (including all viable third parties) have followed this tradition since at least the late 19th century, although there is no law requiring political parties to have a platform. For example, the Progressive party, the Libertarian party, and Green Party, and the Dixiecrats have all followed essentially the same process in adopting their platforms, although the details of appointments to the relevant committees and parliamentary procedure differ modestly between the various parties.
The platform is drafted in a platform committee staffed in accordance with the internal rules of the party, and then is presented to the entire ranks of convention delegates for review and approval.
All major political parties in the United States also adopt a separate state political party platform at a state convention every two or four years.
Platforms are not legally binding on elected officials, but represent an articulation of the party's identity that elected officials are expected to honor by default unless they are making a point of differing from the "party line" on a point.
It is entirely possible that small small political party in the United States, somewhere at some time didn't bother to draft a party platform, but I am not aware of any specific instance of that happening. In a small political party that is unlikely to elect anyone to office, or is likely to elect only a handful of officials, the drafting of the party platform is often seen as the primary mission of the party since it articulates the ideas that are associated with the party and provides a marketing document for the party and for its ideas.
Political scientists have noted that many third parties that had strong political support but not enough to elect many official to office have often found planks of their party platforms migrate to the platforms of major parties later on as the major parties seek to do so to co-opt support from third party voters and backers.
Party platforms are generally not modified between national conventions, because it is not necessary to do so, because they are not binding. I would not be surprised, however, to find some extraordinary process in the rules of some political party in the United States by which a platform could be changed on an interim basis. I know of no case where this has actually been done, however.