Time Machine tells me to start a new backup

Recently I changed the motherboard in my MacBook Pro because of a crash in the graphics card. When they gave me my computer back Time Machine has been seeing it as another machine, when it has the same hard drive (which is the one I want to back up). How can I tell my Time Machine that it's the same computer and to add on to the backups I had before I sent it to be fixed instead of creating a new backup from scratch?