This is probably a stupid question, but is there a way to put these changes in a template specific file or something so we don’t have to change this every time Magento has a new update?
I know this one is a little old, but just in case someone is searching for this answer and wants to know the (possibly more proper) way to do it, open up this file in a text editor:
I know this is an old post, but another good way of making sure that local edits will stay in place is not to edit in the core, instead duplicate the exact path in code/local and edit the file in there.
So for example in this case you would download
app/code/core/Mage/CatalogInventory/Model/Stock/item.php
and create this path:
app/code/local/Mage/CatalogInventory/Model/Stock/item.php and make changes here.