Is there an idiom/word/proverb that says not to change a perfectly good thing or something that works?
The expression "gilding the lily" means to add needless changes to something that's already of high value or near-perfect. So "don't gild the lily" would perhaps covey what you're looking for.
Or, "don't wreck a good thing" might fit, if the proposed changes were obviously going to lower its value. But this is very commonly used in cases where people are benefiting from a system which is non-standard or sub-optimal. Like if you found you were undercharged for a retail good, and were proposing to bring the error to the shop's attention so they could charge you the correct higher value, a friend might say "don't wreck a good thing". So similar in intent but perhaps not exactly what you're describing.
I'd go with "don't gild the lily".
"The great is the enemy of the good."
When you have something that is good enough, do not risk making it worse, just because you are tempted to try to make it great.
Variations are often seen, such as, "The perfect is the enemy of the good." See also: https://en.wikipedia.org/wiki/Perfect_is_the_enemy_of_good
Don't change horses in midstream is another one. Sometimes seen as "swap" instead of "change", or "midrace" instead of "midstream".
"Don't mess with success"
"Don’t mess with success” is another way of saying “if it ain’t broke, don’t fix it.” “And I might say you don’t mess with success” was cited in print in 1964. It does not appear that any one person coined or popularized the simple rhyme.