Is there any other work around to lock sub site Access apart from changing all groups permissions manually in Sharepoint 2010 ?

Sharepoint 2010
 
Hi,
 
We are currently migrating some big sites for user. Current user requirement is to migrate some big sub sites as top level site collection in target.
 
After migration we need to lock source sub sites to avoid any new data entry however as these are sub sites there is no direct option available to lock the access.
 
I know below possible work around
 
1. Stop permission inheritance on sub site and change the permission give to users & group to read only however this has drawback. Once I change permission on source site , it becomes difficult to validate the target site against source if there are any permission issues. Also in case migration failed , rollback process to old site becomes tedious if there are large number of users and groups on site.
 
any suggestion on this will be very helpful.
 
Thanks,
 
suchita