There won't be any serious problem. The course will go on working.
1. The only potential problem is this. Suppose your course is set to show 73 topics in your modified version of Moodle.
2. Then you upgrade to an unmodified version of the code. The course will still have 73 weeks. Fine.
3. Now if you edit the course settings, 73 will not be one of the options on the form, so instead the weeks form field will change to something else.
4. Now, if you click 'Save' your course will no longer have 73 weeks.
However, if you re-tweak your code before step 3, you will be fine.