Hi,

If function is written correctly it will work fine in both form and in tree, i guess you might not have handled records correctly in the compute function and thus when you have multiple record in tree view, you get the value wrongly. 

So it is better to share your code along with the question and fix it.

If currently the field is store=True and then if you need to remove it, just set store=False and then upgrade module and vice versa. No need to uninstall and install.

Thanks