You Can't Take It with You
I suppose if your company were small enough, IT relocation might involve just pulling the wires out of the wall and reusing it. But it wouldn't be easy, and would that cable compare to the cable that's available now? Chances are that cable in the walls from seven or more years ago is Category 3 (CAT3), rated to support 10MB. The standard for the past five years has been CAT5 (100MB); available now and coming soon as the new standard is CAT6 (1000MB). Don't start with old technology to save a few dollars; it will cost you more dollars in the future if you limit yourself now. Since most cable installs are considered permanent, you really need to start from scratch.
Start with your architect's to-scale blueprints for the new office building. If you'll be occupying an existing building, getting blueprints might be a challenge, but it's a worthwhile goal. Copies of those prints sent to cabling contractors can get you accurate quotes. Because cabling contractors install cable by the foot, you really need to give the contractor to-scale prints to measure from.