They'll change your phone number to theirs and replace you in whatsoever group you belong.
In order to prevent such, follow these steps:
- Go to settings on your WhatsApp
- Click on Account,
- Click on Two-step verification,
- Enter pin,
- Input your email address
- And save, that's all.
If someone wants to change it, WhatsApp will ask for the Two-step verification and without it the account won't open.
Let's be cautious and safe.
For security purpose, it is also advised that all your social media accounts should have Two-Step Verification.