Has anyone found IRS proposed changes to an old tax return incorrect? We received one and am trying to figure out whether to challenge with help from a CPA.

Of course the IRS sometimes makes mistakes.  Without some details from you, no one here can guess whether you should challenge what the IRS is saying about your prior year return.  What does the letter say they are changing?  TurboTax does not receive any information from the IRS about your tax return after it has been filed, so no one at TT knows what the IRS is saying to you.

xmasbaby0 is correct - if you believe that the IRS's position is incorrect, you should challenge it or at the very least, discuss it with a tax professional.

Sometimes the IRS makes a mistake because through no fault of theirs, they don't have all the information needed.

Whatever you do, please respond to the IRS in a timely fashion (generally in about three weeks), even if you haven't finished collecting the facts yet. Note that an Enrolled Agent or a CPA can represent you in the discussion with the IRS and, depending on the facts of the case, may even negotiate for an abatement (waiver) of penalties and/or interest even if you end up owning the tax.

Enrolled Agents are regulated directly by the IRS; you can find one near you at https://taxexperts.naea.org/ . CPAs are regulated by each state; you can do an Internet Search on "CPA State Board" and your state's name - every state surely has a way to look up local CPAs.