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In real life, twins that merge into a single embryo in utero is called a chimera, and has two sets of DNA, one from each original twin. When this happens in horses, the foal will express a distinctive brindle coat. As far as the terms go, there's no difference really. "Brindle" is just the name for the type of striping that chimeric horses express. But in other animals, like dogs particularly, "brindle" is just a regular type of coat color. It doesn't mean that the dog is chimeric like it does with horses.

But in the game, "brindle" was the old way that chimerism in horses was handled genetically. The horse would have just one set of DNA, "brindle" would be added to its color name, and some striping added to the coat. This wasn't as realistic because a real chimeric horse has two sets of DNA. So "brindle" horse genetics was updated to the new "chimera" horse genetics, which gives horses two sets of DNA, and the striping comes from those two different coats expressing at once. You've probably seen that striping isn't the only type of chimera in the game now. There are also horses that show a patchwork of the two different coat colors.

This thread shows a few good examples:
https://www.huntandjump.com/forum/discussion/52241/types-of-brindle-patterns

When it comes to changing brindles into chimeras yourself, I don't think you have to. You can choose to keep the old kind if you like or change them to the new kind. But it can't be reversed, so be sure you want your brindle to become a chimera before you upgrade it.

That is not quite correct.

A chimera is any individual with the DNA of two different organisms. Sometimes that is from merged twins but not always the case. Sometimes but not often chimeras can cause brindle markings. Most types of chimeras are invisible.

Brindle is a phenotype. A strippy coat pattern. There are a few things that can cause brindle markings in horses including chimerism, genetic brindle, and somatic mutations/mosaicism.

Chimerism is the state of having two sets of DNA. Not all chimeras are brindle. Brindle is a stripy coat. Not all stripy coat horses are chimerias.