first one implies that the change is almost non existent and the second one there is change and it is existent (but not very big)

Technically, they are both correct. But since you are not a native English speaker, instead of your first sentence, I would suggest that you use one of the following two sentences instead: "There isn't much that has changed" "Not much has changed". Your first sentence could be used in a specific situation but if used incorrectly, it could sound awkward. I would rather say that your two sentences are similar, not the same.