New bitcoin ppa is available at
https://launchpad.net/%7Ebitcoin/+archive/bitcoinCurrently contains bitcoin-qt and bitcoind at 0.5.0 RC1 but will eventually become the stable channel for bitcoin updates once 0.5.0 is released.
Backup wallet
to install:
Backup walletremove stretch's ppa if you have it
sudo ppa-purge ppa:stretch/bitcoin
add new repository
sudo apt-add-repository ppa:bitcoin/bitcoin
update repo listings and install
sudo apt-get update && sudo apt-get install bitcoin-qt