As a user of the service myself while also working for Vodafone we've been provided with a little more information on the change.
Extending the qualifying period to midnight will mean that more customers will be able take advantage of the proposition, as they've got more time to spend the £5.00 required credit.
In terms of introducing a 60 minute cap on the cross network calls at weekends, this will mean that Vodafone are able to keep the promotion although for more customers and for a longer period of time.
Although it does limit the amount of calls that can be made, call pattern research showed that it wouldn't effect 80% of the customers using Free Weekends.
The calls to landlines and Vodafone mobiles still remain unlimited and if you've got Stop The Clock on your phone, after your 60 minutes of cross network minutes have been used, Stop The Clock will kick in as normal. In addition, text message to all networks remain unlimited.
As I'm using Stop The Clock on my number there hasn't been any real impact on my own calls.