Can we all agree on this?

Partnership to Corporation 

An election to change classification from a partnership to a corporation will be treated as if the partnership contributed all of its assets and liabilities to the corporation in exchange for stock and the partnership then immediately liquidated by distributing the stock to its partners.

For more information, see Partnership Distributions in Publication 541, Partnerships and Property Exchanged for Stock in Publication 542, Corporations.

Corporation to Partnership 

An election to change classification from a corporation to a partnership will be treated as if the corporation distributed all of its assets and liabilities to its shareholders in liquidation and the shareholders then immediately contributed all of the distributed assets and liabilities to a new partnership.

For more information, see Contribution of Property in Publication 541, Partnerships and Distributions to Shareholders in Publication 542, Corporations.

https://www.irs.gov/businesses/small-businesses-self-employed/limited-liability-company-possible-rep... 

The right answer, of course, is to tell the clients to make up their fool minds and stick with what they chose or go down the street to H&R Block.  That's why I don't plan on ever dealing with this problem myself.