Do I have to start a new career after installing Breaking Grounds expansion to Kerbal Space Program?

Are you willing to edit manually (via notepad) your current save?

No? You must create a new career to see the new surface features.

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Yes?

• Open your current save in KSP
• Press Alt+F5 to make a temporary backup
• Close KSP
• Locate your save game file. By default it's in <KSP_installation_folder>/saves/<your_career_name_folder>/persistent.sfs
• Open in in notepad (not wordpad)

At the beginning of the file you'll see something like

ROCSeed = -1

Change it to

ROCSeed = 1260332081

Sidenote: this is the value of one of my saves, this means we'll see both the same surface features at the same place, for example a quartz in near the mountains near the KSC

• Save and launch your save in KSP
• If something goes wrong, replace persistent.sfs by your backup save by renaming it.

No, but you might want to.

Depending on how far in the career you are you might have missed out on some of the early missions from the expansion (like make a simple scan of a boboa tree from Kerbin's grasslands).

There are also some missions you may do differently with robotics available.

It is simply your choice, nothing breaks by simply starting with the expansion elements half way through your game.