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I apologize for the delay you have experienced with receiving an answer, but in all honesty, the question value is on the "low" side for the level of knowledge and expertise required to provide an answer.
In any event, you would file Form
8832 to change the entity classification from partnership to corporation as well as Form 2553 to elect subchapter S status.
While there are time constraints, the IRS is usually very flexible about granting "late election relief." For example, you can file Form 8832 and Form 2553 to take effect for tax year 2015. You will also then attach copies of both forms to Form 1120S which is filed for the S corporation for tax year 2015.
Please let me know if you require further information or clarification.
Thank you and best regards,