If I have my coins in cold storage, what's the procedure if I want to sell off my Unlimited coins? I assume it's the following:
- Export my private keys from my cold wallet (or wherever they are stored)
- Download and install Bitcoin Unlimited client
- Import my private keys into my Bitcoin Unlimited client
- Send my BU coins to an exchange (say shapeshift.io) in exchange for Core coins or fiat or whatever
- Provide new cold storage addresses to receive the new Core coins
Is that right?
No. Your BU transaction can be replayed on the Core chain, with the result that you lose all your Core coins.
Ok. What's the best way to accomplish it then?
Get yourself some dust from a block mined on one of the chains. Combine that dust with your existing coins by sending a transaction to another address that you control. Since those coin base rewards don't exist on the other chain, this transaction will only be valid on one chain. After that, send your coins on the other chain to a new address that you control (extra measure just to be sure). At this point, your coins will be effectively disassociated between the two existing chains.
There is another method which may become possible if the second chain is a Bitcoin Unlimited chain. Since blocks will be larger on the Bitcoin Unlimited chain (that's the point after all), they will allow for many, many more transactions. So, you can just start sending your bitcoins to yourself over and over and over again. Over time, these transactions will be written into the Bitcoin Unlimited chain, but will be a chain of unconfirmed transactions on the original Bitcoin chain. At this point, you try a "double spend" to yourself (to a different address under your control) with a higher fee on the Bitcoin chain. Once one of these "double spend" transactions make it into a block on the Bitcoin chain, these coins will effectively be disassociated from the Bitcoin Unlimited chain.
Most probably, the safest method is to use an exchange that lists both coins, and has provisions to do the split for you, that is, to send the "unified old coins" to an address attached to you on the exchange. The exchange will then give you IOU of both types of coins. You can trade those (for instance, selling off your bitcoinx and buy more bitcoin, or selling off bitcoin to get more bitcoinx). When you will withdraw the coins, they will be 1 type only.
But be sure you use an exchange that knows how to handle this. Most big exchanges learned this during the ETH/ETC split. Some made big mistakes. Some stole the ETC of their customers, pretending they didn't "list" ETC.
With ethereum, there was a smart contract that could do the split for you, but with bitcoin, that's not the case, and the only way to do so is to use "newly mined dust", which is hard to come by for a normal user.