From the IRS:

Sole Proprietors

You will be required to obtain a new EIN if any of the following statements are true.

• You are subject to a bankruptcy proceeding.
• You incorporate.
• You take in partners and operate as a partnership.
• You purchase or inherit an existing business that you operate as a sole proprietorship.

From that same page:

You will not be required to obtain a new EIN if any of the following statements are true.

• An LLC that already has an EIN chooses to be taxed as a corporation or as an S corporation.

Call IRS Entity Control 

https://www.irs.gov/businesses/telephone-assistance-contacts-for-business-customers