What have warriors that would typically bear a coat of arms done to their heraldic achievements to indicate they have deserted or defected?

Warriors didn't change their coats of arms, if any, when they changed their allegiance. Coats of arms were considered to be hereditary rights.

You should read about the famous case of Scrope v Grosvenor in 1385-1390, when the first Baron Scrope of Bolton and Sir Richard Gosvenor both used the same coat of arms, "Azure, a bend or".

https://en.wikipedia.org/wiki/Scrope_v_Grosvenor

In the end, Grosvenor was forced to give up "Azure, a Bend Or", and had to take a new coat of arms, which he chose to be "Azure, a grab or". And the Grosvenor family didn't forget about their old coat of arms.

A thoroughbred racehorse, born in 1877 and owned by Hugh Grosvenor, 1st Duke of Westminster, was named Bend Or in allusion to the case. It won The Derby in 1880.

The horse was named 487 years after losing the case.

The 1st Duke's grandson, Hugh (1879–1953), afterwards 2nd Duke, was similarly from his childhood and in adult life known within family circles as "Bendor".[7] His wife Loelia wrote in her memoirs: "Of course everybody, even his parents and sisters, would normally have addressed the baby as "Belgrave" so they may have thought that any nickname was preferable. At all events it stuck, and my husband's friends never called him anything but Bendor or Benny".[8]

And he was nicknamed even longer after the case.

Bendor Gerard Robert Grosvenor (born 27 November 1977) is a British art historian, writer and former art dealer. He is known for discovering a number of important lost artworks by Old Master artists, including Sir Peter Paul Rubens, Claude Lorrain and Peter Brueghel the Younger.1 As a dealer he specialised in Old Masters, with a particular interest in Anthony van Dyck.

The name Bendor is derived from the Grosvenor family's medieval heraldic shield, a bend or, a golden bend (diagonal stripe), which they used until 1389, when it was claimed instead by the Scrope family in the case Scrope v Grosvenor. The 2nd Duke of Westminster was nicknamed "Bendor".

https://en.wikipedia.org/wiki/Bendor_Grosvenor

And he was named 577 years after the family lost the coat of arms of "Azure, a bend or".

After about 1200 or so, an armiger would have been very reluctant to change their coat of arms. If they were in hiding as fugitives, they might display as false coat of arms to avoid detection, and sometimes they would use a different coat of arms at a tournament or something if they wanted to keep their identity secret. A night who was famous might want to disguise himself so that other knights wouldn't be afraid to go against them and lose in tourniments.

But every armiger would want to hand down their coat of arms to their descendants. They wouldn't voluntarily change their coat of arms. The coat of arms belonged to them and their descendants, and represented them and their descendants, not their overlord or their overlords overlord, or their king or their country.

There is also the practice of a higher authority creating an abatement of a coat of arms, making it less honorable, though that was more common in theory than in fact.

An abatement (or rebatement) is a modification of a coat of arms, representing a less-than honorable augmentation,1 imposed by an heraldic authority (such as the Court of Chivalry in England) or by royal decree for misconduct. The practice of inverting the entire escutcheon of an armiger found guilty of high treason has been attested since the Middle Ages and is generally accepted as reliable, and medieval heraldic sources cite at least one instance of removing an honourable charge from a coat of arms by royal decree as an abatement of honour. Other abatements of honour implied by the addition of dishonourable stains and charges, appearing in late 16th-century texts, have never been reliably attested in actual practice. Additionally, as many heraldic writers note, the use of arms is not compulsory, so armigers are more likely to relinquish a dishonored coat of arms than to advertise their dishonor.

https://en.wikipedia.org/wiki/Abatement_(heraldry)#:~:text=An%20abatement%20(or%20rebatement)%20is,by%20royal%20decree%20for%20misconduct.