I had build the Fighting Pit improvement on some of my planets. However, they weren't upgrading to the Arena after I researched Tournaments. I took a look at the ImprovementDefs.xml and found out why. The UpgradesFrom-tag of the Arena was set to the XenoAmusement-tech instead of the FightingPit-improvement. Several of the other approval improvement had a similar error.
Stadium: <UpgradesFrom>Entertainment Network</UpgradesFrom> should be <UpgradesFrom>EntertainmentNetwork</UpgradesFrom> [fixed]
MegaResort: <UpgradesFrom>Amusement Park</UpgradesFrom> should be <UpgradesFrom>AmusementPark</UpgradesFrom>
Arena: <UpgradesFrom>XenoAmusement</UpgradesFrom> should be <UpgradesFrom>FightingPit</UpgradesFrom>
Slaughtertorium: <UpgradesFrom>TournamentTech</UpgradesFrom> should be <UpgradesFrom>Arena</UpgradesFrom>
HuntingGrounds: <UpgradesFrom>SaughtertoriumTech</UpgradesFrom> should be <UpgradesFrom>Slaughtertorium</UpgradesFrom>
MeditationCenter: <UpgradesFrom>Healing Pools</UpgradesFrom> should be <UpgradesFrom>HealingPools</UpgradesFrom>
I haven't checked, if other improvements are affected too.