How does the career mode change when you have unlocked the entire tech tree?

I've nearly unlocked all the science tech tree. How does the game change after that?

Specifically, is there any reason for more science when I've unlocked everything?

I've unlocked pretty much everything within Kerbin's SOI, is there any reason for conducting interplanetary science? Surely, I could fill out the entire tech tree with just 1 more planet. Why, then, would I do science all the way out to Eeloo if I've filled the tech tree? Is there an achievement, more parts, upgraded parts, anything?

Solution 1:

Currently (0.23), there is few practical difference between sandbox and career mode after you unlocked the whole tech tree. You have all parts available and are free to use them to do whatever you like.

The only difference is that you can still use experiments in career mode, while in sandbox mode all experiments are disabled. This doesn't give you any gameplay benefit, but it gives you the opportunity to read the flavor texts of the experiments performed on outer planets and moons. Some of them are quite interesting and/or entertaining. One EVA report on Eeloo is really cute, by the way.

Currently there is a lot more science to farm in the KSP solar system than you need to unlock the whole tech tree. And considering that it is planned that more planets than just Kerbin, Mun and Minmus will gain different biomes, the science output will just increase. If and how the developers will balance this in future updates (more tech tiers? Make later tiers more expensive? Scale down scientific values? New uses for science points?) is currently just guessing.

Solution 2:

If you are using mods, like KSP Interstellar, playing career over sandbox has one advantage: there are more things you can spend Science points on, not only unlocking tech tree. You can upgrade your parts with it, too. Being able to collect them with core Kerbal experiments, not only with Science Lab is a huge advantage - for example, on Kerbin orbit lab produces only 0.017 science per day - you can harvest way more just by observing goo pretty much anywhere.