I tried it the classic way, using the ftp to upload the theme and the new plugins. It seams to be working. I did not overwrite the theme, but saved it under a different name.
Well, I did exactly as suggested: downloaded the new version of the theme, installed the Easy Theme and Plugin Upgrades plugin, tried to upload the new theme having selected the "Upgrade existing theme?" with YES, but I cannot get it to actually upl…