I tried this earlier on in the week - -I am on the 256 jobbie - - and it said you couldn't do it at that time. I spoke to tiscali and they aid the same thing.
I got a letter from tiscali only yesterday saying that my service would be upgraded, free of charge, to the 512 service. I pay £19.99 right now for 256 - - so this kind of money will get me 512, or stay on 256 and pay £15.99. All this in March - - -which, spookily enough, is 12 months after signing up to them.