You have a number of issues here that need to be addressed:

• First, the partnership terminated when it became a SMLLC and the final tax return was due on the 15th day of the third month after that terminating event.
• Based on the above, you most likely have a late filed form 1065 if you did not extend in the time frame of bullet number 1.
• Then once you are a SMLLC you need to make the election to be treated as an association on form 8832 along with the S election.
• There are missed S election rulings, but not sure about the form 8832 not being filed along with the form 2553.
• Unless you can find a ruling that fits your facts, a private letter ruling is expensive