The old logo is still the logo for the Steem blockchain. The new logo is for Steemit.com, which is a front end for the Steem blockchain.
Steem != Steemit.com
Many people get this confused, so the branding should make it more clear. You can still use the "old" logo because it is not really old.
Thats good because the front end is getting worse imo and i like the old logo better.
:D