First - no you can't say
I've changed my heart (unless possibly you've just had a transplant
) To
have a change of heart is a fixed idiom, meaning to change your feelings or attitudes towards something and do something kinder or more positive (from the listener's point of view) :
The boss told us we'd have to work Saturdays, but then she had a change of heart and decided to close.
Notice then that the meaning is slightly different from to
change your mind - which simply means to make a second, different decision or to change your opinion, but with no "moral" implications :
I know I told you we were having fish for dinner, but I changed my mind. We're having chicken. I thought "Catcher in the Rye" was a great novel when I read it at 16, but then I read it again at 22 and changed my mind.
So -
to change your mind can be used to describe any overturned decision or change of opinion.
To have a change of heart is used to describe a change of decision or attitude, but implies that the new decision is in some way kinder or more ethical than the old one.
You can use both
change of mind and
change of heart with the verb
have :
Vodafone has a change of mind on the Galaxy Tablet
Now, however, the company has apparently had a change of heart
but only
mind can be used with
change as a verb :
I've changed my mind about John. I liked him at first but not any more.
Hope that clarifies.