The blade assembly is compatible with all OmniBlend jug sizes, and can be replaced easily. The blade assembly should be replaced when the jug shows sign of leakage (while still being securely fitted), or when the teeth of the bearing are worn.
Along with replacing the blade assembly, always check the condition of the drive socket. If the teeth of the drive socket are worn too, this part should also be replaced. You can find the drive socket here.