Thanks to my colleague Enda O'Connor, who has made p7zip available for Solaris 8 SPARC, it's now possible to upgrade directly from Solaris 8 SPARC to the latest Solaris 10 Update releases such as Solaris 10 5/08 and Solaris 10 10/08. See http://sunsolve.sun.com/search/document.do?assetkey=1-9-250526-1 and http://sunsolve.sun.com/search/document.do?assetkey=1-61-72099-1 for details.
Previously, due to the lack of p7zip on Solaris 8, customers needed to perform an interim upgrade to Solaris 9 or an earlier Solaris 10 release before upgrading to the latest Solaris 10 release.