genesis.json

{
  "difficulty" : "0x200",
  "extraData"  : "",
  "gasLimit"   : "0x8000000",
  "alloc": {
     "0x87C03Cc1F1a0BfABa24c23e929fFc61b6BB0d580": { "balance": "20000000000000000000" },
     "0x8f9b540b19520f8259115a90e4b4ffaeac642a30": { "balance": "20000000000000000000" }
  },
  "config": {
        "chainId": 15,
        "homesteadBlock": 0,
        "eip155Block": 0,
        "eip158Block": 0
    }
}

in genesis.json, change the "alloc" accounts to your accounts
copy current keystore directory to ./chaindata
you could also create new accounts, but these will have 0 balance

create genesis.json in your home directory

cd ~
mkdir chaindata
geth init ./genesis.json --datadir ./chaindata
geth --rpc --datadir ./chaindata --rpcport "8545" --nodiscover --rpcapi="eth,web3,personal,net,miner,txpool" 

in another terminal:

geth attach --datadir ./chaindata
> miner.start()
> personal.unlockAccount(eth.accounts[0], null, 600000000)
> eth.defaultAccount = eth.accounts[0]

/Users/danielbruce/Library/Ethereum/testnet

/Users/danielbruce/Library/Ethereum/testnet is where your existing keystore might be.

create genesis.json in your home/Ethereum directory

cd ~/Ethereum
mkdir chaindata
geth init ./chaindata/genesis.json --datadir ./chaindata
geth --rpc --datadir ./chaindata --rpcport "8545" --nodiscover --rpcapi="eth,web3,personal,net,miner,txpool" 

In another terminal:

geth attach --datadir ~/Ethereum/chaindata
> miner.start()
> personal.unlockAccount(eth.accounts[0], null, 600000000)
> eth.defaultAccount = eth.accounts[0]