I am trying to associate my iPhone and iPad to my "new" computer that has been rebuilt, but I do not want to lose the content of my devices to do so.
I have no access to my "old" computer as this was it and I did not have a full back up?
iTunes Store- Associating a device or computer to your Apple ID
so that has my iPad and new PC, but not my phone (all setup with the same Apple ID) :/
See Recover your iTunes library from your iPod or iOS device.
Retrieving data ...