It is highly improbable there will every be a genuine need to make "Bitcoin 2.0". The Bitcoin network can be modified over time to incorporate new features, and improvements. This evolving software model makes the need for a "clean break" unlikely. In a similar fashion that while the internet evolved far beyond its original concept, we never turned off the "old internet" and started a "new internet".
To answer the question I will assume a new block chain is needed. There is a method to make a clean switch with minimal legacy entanglements.
1) The bitcoin community should determine the new Bitcoin protocol (BTC2) and at the same time reach a consensus or at least super majority on when the current Bitcoin should end (BTC1). It would be possible to modify current Bitcoin protocol to allow stakeholders to vote based on the number of Bitcoins they hold at the time of the proposal. Any proposal to make to new block chain should have widespread support to avoid a scenario where it fragments the Bitcoin economy.
2) A omega block (last block) would be announced after debate and voting. Any bitcoins received after that block would not be able to migrate to the new block chain. Users should take care to not accept Bitcoins after the omega block. To avoid chaos and disruptions this should be announced well in advance, possible issuing new clients which warn users and even reporting post-omega transactions as bad (no value).
3) After the omega block is signed it will be reduced. The only data relevent at to the new chain is the current location of every single bitcoin. This means saving all active public addresses and their current value from the existing block chain. This will result in a significant reduction in size of block chain as the history of coins is no longer relevent, only their final location is. Likewise any public address with a value of 0 BTC doesn't need to be saved.
4) Either include the reduced block chain or a cryptographic hash in the genesis block. This provides a bridge between the old network and new.
5) The new block chain now has all the required information to migrate coins as needed by their owners. Since the BTC2 network won't "know" of transactions after the Omega block anyone coins transfered after that will have no value in the new network and thus no value in old network. It won't be possible to force BTC1 to stop as it is a P2P network but it is unlikely there will be much demand for unofficial post omega "orphaned" BTC1 transactions.
6) To avoid complicating the BTC2 protocol there will be a single transaction possible w/ BTC1 addresses/coins called "TRANSFER". The TRANSFER transaction will take a BTC1 public address and BTC2 public address and the transaction will be signed by the BTC1 private key. Thus the "owner" of any coin at the time of omega block will be the only one who can transfer it to the new network.
7) Once coins are transfered to BTC2 network via the TRANSFER transaction the BTC1 public/private key and wallet have no value or use and can be deleted by the owenr.
8) During the transistion it will be necessary for wallets to have a copy of reduced BTC1 omega block chain so they are "aware" of BTC1 and TRANSFER protocol. However once transfered a user will never need a "migration" wallet so eventually versions which drop all legacy support (TRANSFER protocol & omega block chain) will be released.
9) Nodes could choose at anytime to be "legacy free" and will ignore TRANSFER transactions and likely as time passes many nodes will stop accepting these legacy transactions as the volume of coins transfered will continually decline. As long as one node supports legacy addresses and TRANSFER transactions it will be possible to eventually have those transactions confirmed albeit with significantly increased confirmation times.