British friends - help, please?

I desperately need a piece of info - have you changed your money lately? Mum and I have had some British money left from our last visit to Cambridge (altariel and the_wild_iris might remember the events), but since it won't be enough for our trip in August, we decided to change some more.

Now the twenty-pound note we got is different from the ones we had from last time. Not much, but it's different. So, is the older version still valid, and if not, will the bank exchange it for the new one? Because if they don't, then we're in serious trouble.

So - info, please? Pretty please? Mum is seriously panicking (this is a lot of money for us), and I'm getting nervous as a result, too.