The Merge Contacts section has two options which allows you to be able to swap the ID numbers when merging contacts.
NOTE: Infusionsoft does not allow you to change the ID numbers when merging Contacts. Although I do not why this is not allowed, because it does not matter when merging contacts via the API. Swapping the ID numbers has no affect to the merging of the Contacts. If you in doubt, then trying merging some dummy contacts first.
Allow Contact IDs to be Swapped – When turned on, it will allow you to swap the Contact ID numbers.
Convert Contact ID to a Link – This is On by default, which when you hover your mouse pointer over the ID number, it will show the Contact Record.