Me: woohoo! this yum downgrade thing sounds like just what I need

Also me:
yum downgrade helix-p4d
Loaded plugins: downloadonly, fastestmirror, presto
Setting up Downgrade Process
Loading mirror speeds from cached hostfile
Resolving Dependencies
--> Running transaction check
---> Package helix-p4d.x86_64 0:2018.1-1738923 will be a downgrade
--> Processing Dependency: helix-p4d-base = 2018.1-1738923 for package: helix-p4d-2018.1-1738923.x86_64
--> Processing Dependency: helix-cli = 2018.1 for package: helix-p4d-2018.1-1738923.x86_64
---> Package helix-p4d.x86_64 0:2018.2-1740258 will be erased
--> Finished Dependency Resolution
Error: Package: helix-p4d-2018.1-1738923.x86_64 (Perforce)
Requires: helix-cli = 2018.1
Installed: helix-cli-2018.2-1740258.x86_64 (@Perforce)
helix-cli = 2018.2-1740258
Available: helix-cli-2016.2-1598668.x86_64 (Perforce)
helix-cli = 2016.2-1598668
Available: helix-cli-2017.1-1738874.x86_64 (Perforce)
helix-cli = 2017.1-1738874
Available: helix-cli-2017.2-1611521.x86_64 (Perforce)
helix-cli = 2017.2-1611521
Available: helix-cli-2018.1-1738923.x86_64 (Perforce)
helix-cli = 2018.1-1738923
Error: Package: helix-p4d-2018.1-1738923.x86_64 (Perforce)
Requires: helix-p4d-base = 2018.1-1738923
Installed: helix-p4d-base-2018.2-1740258.x86_64 (@Perforce)
helix-p4d-base = 2018.2-1740258
Available: helix-p4d-base-2016.2-1692401.x86_64 (Perforce)
helix-p4d-base = 2016.2-1692401
Available: helix-p4d-base-2017.1-1738874.x86_64 (Perforce)
helix-p4d-base = 2017.1-1738874
Available: helix-p4d-base-2017.2-1721882.x86_64 (Perforce)
helix-p4d-base = 2017.2-1721882
Available: helix-p4d-base-2018.1-1738923.x86_64 (Perforce)
helix-p4d-base = 2018.1-1738923
You could try using --skip-broken to work around the problem
You could try running: rpm -Va --nofiles --nodigest