If I understand, you are re-feeding an 800 amp circuit up to an existing point and leaving the rest of the circuit as is. The existing EGC was too small to start with.
I believe you are doing the right thing by using properly sized EGC for the new portion. As far as the existing portion, it was wrong to start with, and the issue is not clear as to whether or not you should do anything with what portion was existing. I say if it is easy to replace then by all means replace it. Chances are it may not be a simple fix otherwise you may not be asking.
One simple fix would be if you have a qualifying metallic raceway to use as the EGC, then all you need to do is disconnect the existing conductor.