No idea what the book and holdability means re: S fare upgradability. My best advice is to book by phone and carefully confirm upgradability with the agent. This worked well for me recently upgrading S to C on EWR/LHR/EWR (see http://www.flyertalk.com/forum/Forum50/HTML/015275.html ) For better or worse, some fares that are not supposed to be upgradable really are, some that seem they should be are not, some are upgradable only by 1Ks using SWUs, some clearly state what's what in the rules, some don't.