I spoke to the Platinum service desk this week and it turn out it is quite simple (as some people have already mentioned in this thread).
You are eligible for Partner Gold in either of the following 2 situations:
1. You earn 500 status credits after being upgraded in Platinum.
2. You earn 500 status credits after passing a review date if your membership level stays the same.
In other words, ignore all your other status credits balances or requirements. It is as simple as earning 500 additional status credits in one of these two scenarios. (The key here being additional status credits)
In my case, I recently hit ~1300 Status Credits which means I was upgraded from Gold to Platinum. However, the extra 300 status credits above 1000 were effectively earned before my upgrade so they do not count towards partner gold. I have to earn 500 additional status credits, i.e. reach 1800 SCs , to get partner gold.
Therefore, the process for me is like this. Earn Platinum at 1300 SCs, Earn parner gold at 1800 SCs (+500) , earn platinum renewal at 2100 SCs (+800), earn partner platinum at 2600 SCs (+1000).
Parter Platinum is the same but require 1000 additional status credits. Another thing to keep in mind, I believe partner status is tied to your own, so you would need to re-qualify to keep it.