Time capsule cannot see device

[tonydavis]

I had to change my router and with it all network settings. As a result I could not "see" my time capsule in airport. So I reset it.
Airport could then see it. I re-set it up again with new network details etc.
So now if I look at the time machine backup it can see all my previous backups. BUT when I enter the utility to continue with backups, it refuses to let me use my "previous drive" and asks me to use a "new drive" that it has set up - which says no existing backups on it.

So how do I make it see both drives as being the same thing? Or must I start backups all over again?