the difference is your rear sag is now reduced so the bike sits higher at the back tipping weight forward. i added an article on spring weight to
upnorth, there is a table from section 8 ( no longer on their website
) which has suggested springs/rider weight. IMHO for bumpy UK road use all the weights quoted are slightly on the high side but it is a good starting point. somewhere in that article it also gives the max spring change possible without a revalve.
I'm 145lbs and run a 70 spring, I've got 64 and 75's which i have tried but prefer the 70 for my usual roads.