My home planet has been totally colonized. Buildings are on every square, upgraded as far as they can go. It's now producing an extra bit of food. Can I push this food off-planet to encourage faster growth elsewhere? Or does this additional food just get wasted, now that I can no longer use it to grow my population? If so, I'd prefer to replace the farms with more long-term productive buildings. Which will happen eventually, but I can always bootstrap another colony before that comes about.
You could keep a little extra food in case your population move offworld to another colony to speed up the growth as opposite to slow growth at just "one" food.
As far I am aware you can't share food off world. So go ahead switch to long-term production building.
Upgrade your farms and replace some of the old farms with more useful buildings!
You can't currently export the food so unless your population drops and you want to grow the pop back it's wasted.
In patch 1.5, food was converted into an empire-wide resource much like minerals and energy. Thus, having a planet generating extra food is no longer a negative thing necessarily.
A planet that has a lot of food bonuses can help balance out other planets that are heavy with minerals, energy, or science which allows you to specialize planets (particularly those with planet-wide bonuses) a bit more if you wish. Food shortages (where you generate less food than you consume) will subtract from your pool of stored food until you run out of stockpiled food, which will trigger starvation penalties. If you cap out your food stockpile, excess generated food will contribute to population growth.
The economy page at the Stellaris Wiki has some additional details when it comes to managing food.
I like to stay at least 3+ food production on all planets even when they are full.
Do remember though that if anything, like a faction, causes your people to become unhappy at all, and just one of them starts to produce 2 food instead of 3, that now you are in a position that you might go negative which will cause more people to want to join the faction. Spiraling down to death.