As far as i know you cannot change a structure STRING directly with str_cpy or even ="blablabla" the only way i have gotten to change a structures string was by making the structures string equal to a string. Like
STRING* new_string = "blablabla";
STmi.s_mi = new_string;

You can probably do that then alter new_string.
I ran into this problem about 2 months ago and that's how i fixed it. Have it in a project i am working on now and it works well.
could be another way but that's the only way i have gotten it to work without an error.