Use the address on page 4 of the attached instructions:
https://www.irs.gov/pub/irs-pdf/i1065.pdf
Also see the attached from the IRS website:
https://www.irs.gov/individuals/business-name-change
Just write a letter (on business letterhead) to the IRS at the appropriate address noted in the instructions to the 1065 explaining that you changed from a partnership to an LLC in accordance with IL state law. Also note that you will not be making any entity election and will remain taxed as a partnership in accordance with the federal income tax regulations.
Make sure that the letter is signed by a managing member and note that designation below the signature.
It is highly doubtful that the IRS will send you an acknowledgment. However, you can request this in your letter indicating that the bank would like to see a response from the IRS.
Finally, when you file the 2016 tax return, I would also check the box that the name has changed. Just a double check.
*A reminder that posts in a forum such as this do not constitute tax advice.
Also keep in mind the date of replies, as tax law changes.
