For reasons that aren't fully clear to me, such a reaction from this generation is common, but it then very often turns out that the previous generations weren't that bothered about the spelling. and, as in this example used the other variant.
My Maclennan line were the same, my great-grandfather having used both Mac and Mc during his lifetime, never mind single and double "n", and lower and upper case for the "l", but in the next generation the spelling solidified on
Maclennan, and an aunt of mine refused to believe the tree that I traced on the basis that the spelling was much more often Mclennan than not.
It's possible that the 1911 National Insurance Act led to standardisation of surnames in a way that had never happened before. This measure gave the British working classes the first national contributory system of insurance against illness and unemployment, - there had been occupationally, mainly locally based "friendly societies" and some limited trades union schemes before then, - and required the building of a national database on which there could have been confusion unless people used a consistent spelling (for UK readers, - can you immediately tell me your NI number?

, i.e. you know your name & date of birth better than your NI number). This was the first time that there had ever been such a continuing database. (It's still there, headquartered in Long Benton just outside Newcastle-upon-Tyne, but whether early records may ever be made available is not known, never mind that many early records are known to have been "weeded" from the files. Surviving records are more likely to relate to living persons, so that that could preclude any release of early records.)
This neat summary from
http://www.spartacus.schoolnet.co.uk/Linsurance1911.htm .
All wage-earners between sixteen and seventy had to join the health scheme. Each worker paid 4d. a week and the employer added 3d. and the state 2d. In return for these payments, free medical attention, including medicine was given. Those workers who contributed were also guaranteed 7s. a week for fifteen weeks in any one year, when they were unemployed. These benefits were paid at Labour Exchanges which provided unemployed workers with information on any vacancies which existed in the area.
As regards
Mc vs
Mac it's possible that around this time that the belief arose that the different spellings indicated a different geographic origin, or a different religious background. Neither of these, except possibly for short periods on the geographic side, are correct; but it can well be imagined, particularly in the West of Scotland in the early decades of the 20th century that the religious aspect led to strong opinions.
By the geographic comment I mean that it can be shown, over limited periods of time, - a few decades, - that the one spelling was the more commonly used than the other in a certain part of the country, but never to the extent that one spelling was exclusively used; and never over a period of centuries.
David
PS
The NI Act led to the incident known as the "Turra Coo"....
This from http://www.turriffanddistrict.com/index.php?topicid=4 .....
Now to the world famous Turra' Coo! In 1911, Lloyd George, the Liberal Prime Minister, brought in National Insurance legislation. Robert Paterson of Lendrum Farm refused to comply. He maintained he looked after his employees. To pay the imposed fine, a white cow was impounded, to be sold in the market place in Turriff. The cow was brought to the Square, be-ribbonned and painted with the words "LENDRUM TO LEEKS" a reference to Lloyd George's Welsh roots. A riot ensued! Sherriff's officers were pelted with eggs, soot, neeps, cabbages, divots and kail stumps by a mob of a thousand. The Coo escaped, only to be taken to the Mart in Aberdeen, to be bought by local farmers and brought back to her home at Lendrum, where she died 6 years later. A memorial to her, and the events which led to the formation of the Scottish Farm Servants' Union can be seen at Lendrum Farm.
See also http://www.springthyme.co.uk/wwwboard/messages/153.html for the bothy ballad The Far Famed Fite Turra Coo...