I've played around with the blockchain.info wallet, and here's what it looks like is happening:
Blockchain.info shows the "This Is Your Bitcoin Address" by date the address was added to your wallet (not the date of the oldest transaction on the blockchain)
To determine blockchain.info's "sort order" for your addresses, export a paper wallet (from the button on the "Wallet Home" tab)
The order of addresses on that pdf is the order Blockchain.info will use when choosing your Default Address. You need to archive/remove all of the addresses above your DESIRED address for it to be your default.
So if you want to change the default address you have two options:
If you don't need the 'old' address anymore
Simply transfer the BTC out of the previous address into a new one (either one you already have, or click on the 'create new address' button).
Archive the 'old' address. It's still available in your wallet under the Receive Money / Archived tab.
Archived addresses won't be shown as your main address.
If you want to keep the 'old' address in your wallet
A little bit trickier, you'll be DELETING the address & private key, then re-adding it.
- Generate your paper wallet backup, and have it open. Make SURE you have the private key of the address(es) in front of you.
- Archive the address(es) above your desired address
- Go to the 'Archive' tab and find those archived addresses. Check the box next to them and select DELETE. (blockchain.info will ask you if you're sure. Then they triple check. It's a good idea to make sure that yes - you still have the private keys saved from step 1 above)
- Re-import the private keys taken from your paper wallet) for your addresses. It's under the "Import / Export" tab. When importing, select IMPORT KEYS DIRECTLY (instead of SWEEP KEYS) if you want to keep the same address in your wallet.