Answer: Foxhound's executable code is stored together with Foxhound data inside the Foxhound database, so "restoring the old build of Foxhound" is the same as "restoring the old Foxhound database".

The post-setup process creates a backup copy of your existing Foxhound database, and you can restore that copy by copying the foxhound.db and foxhound.log files as follows:

On Windows 7, copy the foxhound.db and foxhound.log files from:

C:\ProgramData\RisingRoad\Foxhound4\backup\previous_build

to:

C:\ProgramData\RisingRoad\Foxhound4

See also...
How do I backup my Foxhound database?
How do the Foxhound backup shortcuts work?
What is the upgrade policy for installing new builds of Foxhound?