Doubt it'll have worn your cassette out. Changing a chain every -6000km should keep your cassette in order
I wouldnt worry about it and keep the chain you have for another 2000-3000 miles
Why are you changing your chain after only 1000 mile anyway ?
Edit : Oh its worn. Have you been using a new chain on a old cassette , odd its worn after 1000 miles ?
If the OP is saying his chain passes the 1.0 mark then he should get a new cassette too otherwise he will wear out the new chain rather quickly.
Saying that a chain last for 1000 or 3000 miles is misleading advise really..... all depends on the riding conditions. If the rider is a heavy rider or he does lots of hills or perhaps he rides under sandy conditions then the life of the chain would be nothing like you are saying. Even riding habits have an impact on the chain's life. Since I started riding hills off the saddle I think I have shortened the life of my chain dramatically..... maybe a light rider would not notice this very much but I'm 16 stones.
I'd been measuring chain wear with a ruler and one has to be very accurate or mistakes will occur. I changed the chain the other day thinking that I was doing in time to save the cassette but I was wrong. This was the second time I made such mistake so I got myself a tool to do the measuring in the future. I have changed the chain in good time many times but very annoying when you find you made a mistake. My new chain is slipping on my fav gears, 4 sprockets. This was a new cassette and chain that I ruined in just over a 1000 miles.... very annoying. The old chain was working fine so I'm tempted to put the old chain back and ride it until starts slipping.... I might damage the chain ring too though.
So, to be safe, at 0.75 chain the change!!!!