Ah, the old "triangle" - I can do it Fast, Cheap, or Right. I can probably even do any two of the three, but never all three!

The wording of the OP is such that each time I read it I interpret it slightly differently, but I think that basically all of the sub-tenants equipment (including the fiber termination) is being removed from the current space and moving to the new. I would pull new 12-pair or 25-pair cable from the Building Entry, and a new Fiber from the Building Entry rather than extending the existing feeds. Once that is done, fitting out the new space for 7 desks would be the same as if this was a new install.

I'm not clear on the statement about the new office having 4 CAT5 already installed - 450' is quite a distance for ethernet. If it really is between the old and new offices, it could be re-purposed to carry the 7 dial-tone lines, and simply extend the FO. Frankly, you could carry the dial-tone on 3 of the 4, and use a media converter to transition the data across the fourth. But I would still prefer to feed the new office directly from the MDF and eliminate any issues down the road when they decide to make more changes to the existing space.