I have a multiple entry visa from Hungary, but since my plans changed, we are now likely to spend more days in France than Hungary. Will there be a problem?
No. This sort of situation happens all the time, as long as you still go to Hungary, use the visa for the same purposes (EG, tourism), and leave within the validity dates of the visa you won't have a problem.
The answers given here might also give you some useful insight to the visa.
(EDIT: The reason I mention that you would still need to go to Hungary is if you didn't it might look like you applied through that embassy because it is easier to obtain a visa there, when you never planned to actually visit that country, but it is easy to see this is not the case here)
- You can make minor adjustments to your travel plans after the visa is granted. You can even add or skip entire Schengen countries.
- You must not misrepresent your itinerary to get a visa.
- You should avoid giving the impression that you misrepresented your itinerary.
The last point is the possible problem. Do you have documentation why you changed your plans? Just a whim or a specific reason?
Likely not, but it depends...
The rules for Schengen visas say you must apply for a visa from the country that is the main destination for your trip, and names length of stay as the deciding factor for "main destination" if nothing else decides it.
The rules also allows for minor derivations.
And those are the rules for single-entry visas, yours is a multiple-entry. Nobody expects you to have planned every trip to the Schengen area (or elsewhere) in the period a multiple-entry visa is valid, when you get it, so you have more flexibility (and typically only get them when you have a good history).
But any visa can be cancelled if the authorities finds reason to suspect it was fraudulently obtained.
In your case my guess is you'll be fine, it's really only the assumption of fraud you'll have to worry about. - But the decisions are not mine.