By renaming your "old" wallet.dat to wallet.old and then starting Bitcoin Core, you will create a completely new BTC wallet.dat... So, your idea to take an address from the "NEW" wallet.dat, shutdown Core... then rename "new" wallet.dat to wallet.new... rename wallet.old back to wallet.dat... then start Core, and then send ALL your BTC from "old" wallet.dat to the address you just wrote down from "new" wallet.dat then your BTC will be "safe"... it is now on a completely new address, in a new wallet with all new seed/addresses/private keys.
You can then safely import copies of the "old" wallet.dat into BCH Core (and probably BTG Core) to get those coins as all the BTC will be gone... or you can dump the private keys from your "old" wallet.dat and import the old keys into compatible ilghtweight SPV wallets if you don't want to download 3 copies of the blockchain
NOTE: If you're super paranoid (although BCH is fast becoming "worthless" again)... You could then repeat the exercise with BCH so that you create new BCH wallet.dat so when you import into BTG your BCH is "safe" #cryptoInception
Ok, thanks for this as I wanted to make sure my thought process was correct on what is happening, which it seems it is.
I know I probably missed the boat on BCH and I am in no hurry to sell anything, but just thought it would be a good idea to finally get around to separating all these coins from one another so I can be in a better position going forward should one of the prices spike up again. But before I do anything I want to protect my btc which is my main concern for now.
Thanks again for the input, it is what I am looking for.
Load a copy of electrum, do what it says when it is setting up. Like writing down on paper the seed and the phrases, not storing them in a computer.
First save a copy of the bitcoin core wallet.dat. Next....
Generate a Receive address. Then go to Core and execute a Send to that receive address. Wait if you like and verify it worked. Do this until the Core wallet reads 0.00 funds. Now you are done with emptying out the wallet.
Save a copy of the bitcoin wallet and the electrum wallet.
Next you are ready to take the core wallet.dat private keys and use them to retrieve your bitcoin cash.