Upgrades

Every character has three paths, each with six upgrades. Different upgrades have different costs to buy in order to use this upgrade ingame. However, when you get a tier 4 upgrade, you need to take out two tier 1, 2 or 3 upgrades from the two different paths. Say if I have Radioactive Darts, I can't omit the upgrades on that path. I need to omit two upgrades fro other paths. You can choose which paths to omit. Some tier 4 upgrades instantly rule out a tier 1, 2 or 3 upgrade. You can only get one tier 4 upgrade. From that tier 4 upgrade, you can get tier 5 and 6 upgrades.