You cannot change a current membership to another one.
A new membership will stack to start only when the current membership has expired.
You should contact 48 support to see if they can make some sort of exception in your case but I have no idea if they do this for people.
Hope this helps.
Same problem with a friend of mine and it seems unfair that someone who pays €20 a month for membership can't roam but someone who pays €10 can. Why is that? Very inflexible if they can't change it for people who autorenewed before they travelled.