At the molecular level? Do you mean the chemical level?
DNA doesn’t change from it’s basic coding, if that’s what you’re asking.
The DNA that makes a strain of bacteria into that strain remains. (that is, unless you expose the organism to a chemical like alcohol).
Be more specific. What kind of change to the genome is required by evolution, but has not been observed?
Here, for example, are a few of the known kinds of changes to DNA.
When you say DNA can't change it's basic coding, what do you mean, specifically?