From the blog post annoucing their new white paper:
In the first Libra white paper, we announced a path to eventually transitioning the network to a permissionless system. In the months since, a key concern we heard is that it would be challenging for the Libra Association to guarantee that network compliance provisions would be maintained. In the updated white paper, we present the approach we are exploring to offer new entrants the ability to compete for the provision of core network services and participate in the governance of the Libra network while ensuring the Association’s ability to meet regulatory expectations.
Other changes include adding single-currency-backed stablecoins, and it seems like the libra itself will now be backed, in turn, by those single-currency coins.