how to downgrade X server

[Will Parsons]

I purposely held off on upgrading the to new X server for over a year
now for fear of breakage, but yesterday I upgaded to the latest Xorg
server along with the associated video and input drivers and am left
with a black screen upon starting the X server.

Since I can't figure out how to solve the problem, I'd simply like to
go back to the previously installed software, but the documentation is
unclear on how to do so.  I have the saved versions of the previous
packages in /usr/ports/packages, but doing the naive (and apparently
incorrect):

# pkg install /usr/ports/packages/All/xorg-server-1.7.7_13,1.tbz
Updating FreeBSD repository catalogue...
FreeBSD repository is up-to-date.
All repositories are up-to-date.
pkg: /usr/ports/packages/All/xorg-server-1.7.7_13,1.tbz is not a valid
package:
no manifest found
pkg: No packages available to install matching
'/usr/ports/packages/All/xorg-server-1.7.7_13,1' have been found in the repositories

It should be possible to re-install from the saved packages, right?
But how?