[Edit: This is not correct. See posts following. Left in for completeness of thread.]
I am afraid that Word cannot do this all the way for you.
Word's organization of sorting sees 2 as less than 101 and so puts it first.

The index, though, is a field.
When you have your final draft, you can unlink the index field and then rearrange it to suit. Ctrl+6 or Ctrl+Shift+F9 will unlink a field. This changes it into ordinary text which can be edited.
Dealing with Fields in Microsoft Word

To update this, you would need to insert the index again. If only part is changed, your could unlink the new index and just paste the updated part into your reorganized index. If a lot is changed, just delete the old one and reorganize the new one.

You could, instead, change your numbering system. so that instead of 1 and 2 you hand 100 and 200. That may not be within your job description, though.