Question: I'm writing a story about Robin Hood. Some characters have vast changes in their personalities than it is generally known about them. would that be a problem?Answer:
If you mean the characters will be vastly different than they are in other people's stories, that's not a problem at all. Robin Hood is a classic tale that's in public domain, so you are free to use it however you like.
Of course, to gain the support of Robin Hood fans, what you might want to do is make the characters similar enough to be recognizable but heighten or put a twist on the traditional tale so that the reader sees your work as one that gives a new perspective or interpretation of the story, rather than one that wrecks it or eliminates all the elements they like about the original.
It's a tricky balance, because you want Robin Hood fans to like your book and buy it, but at the same time you want to win over a new set of readers that finds the original story lacking - or would find a straightforward retelling rather dull.
You can't please everyone, but you do need to make a story that will please enough.