Is blockchain conversion feasible

[GCInc.]

This is not particularly a bitcoin related question, but concerns all cryptocurrencies;

Suppose there arises the need to dispose of the existing codebase and migrate to a completely different algorithm / protocol (for instance from sha256 PoW to Proof of Stake), how difficult would it be to convert the whole blockchain to a different format in order to retain transaction history?

Naturally the size of blockchain has a large effect, but are there some particularly preventive difficulties instantly to be seen (for eg. sha256 or Scrypt)?

[DannyHamilton]

This is not particularly a bitcoin related question, but concerns all cryptocurrencies;

Suppose there arises the need to dispose of the existing codebase and migrate to a completely different algorithm / protocol (for instance from sha256 PoW to Proof of Stake), how difficult would it be to convert the whole blockchain to a different format in order to retain transaction history?

Naturally the size of blockchain has a large effect, but are there some particularly preventive difficulties instantly to be seen (for eg. sha256 or Scrypt)?

I'm not sure what you are looking for.  The protocol is the only thing that secures the contents of the current blockchain.  If you can convince everyone to switch to a new protocol, then the new protocol can be written to simply accept the existing blockchain transaction history.  Essentially, the entire transaction history of the current blockchain would become the "genesis block" of the new system.

[GCInc.]

Essentially, the entire transaction history of the current blockchain would become the "genesis block" of the new system.

Thus, transaction records up to that point would be lost? That would technically relatively simple, and probably the task of migrating the whole blockchain is not feasible.
 
I was looking for a scenario where for example Bitcoin would need to start using Scrypt or Cunningham chains PoW technology. Just how big a mess would that be, if transactions are pruned would the genesis block retain address balances?

[DannyHamilton]

Essentially, the entire transaction history of the current blockchain would become the "genesis block" of the new system.

Thus, transaction records up to that point would be lost? That would technically relatively simple, and probably the task of migrating the whole blockchain is not feasible.
 
I was looking for a scenario where for example Bitcoin would need to start using Scrypt or Cunningham chains PoW technology. Just how big a mess would that be, if transactions are pruned would the genesis block retain address balances?

Lets try this again...

What would be done with the transaction history would depend on the developers of the new protocol.

The simplest thing to do would be to leave it exactly as it is, with nodes recognizing a particular block height as the point to begin using the new proof-of-work.

It would also be possible to strip all the unnecessary data (blockhash, nonce, etc) out of the blockchain, and accept the new trimmed down blockchain as a transaction history.

If the change is big enough and significant enough, its quite possible that it will just be implemented in a competing crypto currency and people will just transition from one currency to the other over time rather than modifying bitcoin.  It is far too difficult to get a consensus on such a significant change.

[GCInc.]

The simplest thing to do would be to leave it exactly as it is, with nodes recognizing a particular block height as the point to begin using the new proof-of-work.

Ok. And are the blockchains for all different types of coins (sha256, scrypt, proof-of-stake, cunningham chains, what else) moderately similar so adoption of the old blockchain to the renewed type of currency does not pose great obstacles?

It is far too difficult to get a consensus on such a significant change.

Yes, a more likely situation occurs where one of the newer cryptocoins with less inertia come to the conclusion that their setup was baad from the beginning, and needs to be changed extensively.

[DannyHamilton]

The simplest thing to do would be to leave it exactly as it is, with nodes recognizing a particular block height as the point to begin using the new proof-of-work.

Ok. And are the blockchains for all different types of coins (sha256, scrypt, proof-of-stake, cunningham chains, what else) moderately similar so adoption of the old blockchain to the renewed type of currency does not pose great obstacles?

The details matter, so a general answer will probably be insufficient, but I'd suspect that in most cases as long as you could get a consensus on the new protocol there wouldn't be too many technical barriers to creating a client that recognized a particular block as a point to switch to new rules.