Until this week, most YouTube channels that were created before May 2009 - so-called "legacy" accounts - could be moved from one Google account to another. Now YouTube is providing a different option: link your YouTube channel to a Google+ Page, and transfer ownership of that Page to the new owner's account.
If you open the advanced settings (www.youtube.com/account_advanced) of your legacy YouTube channel you should still see the option to "Change which Google Account is linked to this account".
Clicking "Change Google Accounts" takes you to www.youtube.com/my_account_unlink which now prompts you to link your YouTube channel to a Google+ Page.
If you choose to link your YouTube channel to a Google+ Page at this step, you will see a prompt to update your channel identity like this:
If you want to be able to transfer ownership of your YouTube channel to a different Google account, you must link your channel to a Google+ Page.
A Google+ Page and its associated YouTube channel can have up to 50 different managers. After two weeks, a manager of the Page can be promoted to owner. Ownership of the YouTube channel will be transferred along with the Page.
So what's the catch?
• After 14 days the link between your YouTube channel and its Google+ Page becomes permanent.
• You will not be able to link your YouTube channel to a Google+ Profile on a different Google account.
• Also be aware that Google+ Page name changes are limited to 3 changes per calendar year. That means you need to pick a name for your Google+ Page and YouTube channel that you would be happy to stick with for a while.
If you run into trouble you can contact YouTube 1:1 support for assistance.
Options for updating your YouTube channel's account to a Google+ Page.
YouTube Help: Manage channels linked to a Google+ Page
Google+ Help: Transfer ownership of your Page