Williams, 35, lost his place in the Wales team to Sam Warburton during the Six Nations but still harbours ambitions of playing at the World Cup.
Italy number eight Sergio Parisse continues as captain after leading the Barbarians to a 38-32 victory over England at Twickenham last Sunday.
Joe van Niekerk starts again at blindside flanker and Paul Tito continues in the second row, otherwise it is all change in the pack.
Toulon prop Carl Hayman and hooker Sebastien Bruno join Thomas in the front row while Tito will pack down alongside Samoan Joe Tekori, who made such a decisive impact off the bench at Twickenham last weekend.
Clermont Auvergne fly-half Brock James and France international Sebastien Tillous-Borde form a new half-back combination for the Barbarians.
Leinster full-back Isa Nacewa starts at full-back while Seru Rabeni starts at centre and former New Zealand wing Doug Howlett replaces Tim Visser.
Willie Mason, the former Australia rugby league international, will continue his rugby union education off the bench.
Gilbert has released a new line of rugby cleats. The Gilbert Virtuo 8S is part of the exciting new product. Check it out.
The Barbarians are one of the top invitational rugby sides with a long history and classic rugby jersey.
The Nike Tiempo is a solid rugby cleat and one of few styles still made from full-grain natural leather.
The Lions get ready for their matches with this green training jersey. It's what the players wear. Get in the Gear!
A cool looking all black rugby cleat with the high performance adidas is known for. Get in the Gear!
Wear the crest of the British and Irish Lions on your t-shirt. A great look for the summer.
The Lions are ready to get their Australia tour underway. They arrive in Perth on Monday.
The New Zealand All Blacks training jersey for 2013/14. Get in the Gear!
The USA Rugby Pro Alternate rugby jersey is perfect for any fan of the Eagles. Get yours to wear during the summer Test matches.
The NEW All Blacks 2013/14 jersey has arrived at World Rugby Shop. Dare to wear the colors of the All Blacks.