Aside from waiting, you could plan a maintenance window for changing the org wide defaults as suggested in the help docs here:
The purpose of this article is to discuss best practices when making
organization-wide sharing changes. These changes require a
recalculation of all record access levels in your organization and can
cause adverse effects such as:
Contention Lock errors ("The record you are trying to edit is
Group Membership Lock errors ("Group Membership Locked").
Negative Performance Impacts on your organization.
When making an org-wide sharing change, it's always best to plan and
prepare prior to making these changes. The specific changes to do
this for are:
Org-Wide Default change (i.e. Changing default access levels from
Private to Public Read/Write).
Large-scale Role Hierarchy modifications.
Creation of multiple sharing rules.
Large-scale Public Groups maintenance (adding/removing members,
deleting groups, etc).
For all customers, we recommend that the following are done:
Test the change in a full data Sandbox to ensure the change behaves as
Plan a maintenance window during off-peak hours (w/ low user
activity), such as at night, a weekend, or holiday.
For our enterprise customers with very-large organizations, we
recommend the following (in addition to the above):
Plan maintenance windows during a weekend, holiday, or period of
extended low user activity.
Log a Case with salesforce.com Support to evaluate the changes that
are planned to understand overall impact to your org and instance.
This may involve seeking an escalation to the Tier3 team.
Review the following white papers for information on sharing related
features and design concepts that can assist in streamlining your
To better understand what causes the locks take a look at the Record Locking Cheat Sheet here.