how to re-sign an applet after certificate has expired

[MasterOfDisaster]

hi, couldn't find a suitable forum to post this so i chose the newless clubies section...

i have signed an applet a year ago with a purchased thawte certificate which has now expired (done this for a company where a friend of mine works).
now they purchased a new certificate and i'm struggling with the re-signing process.  i can import the new cert into my keystore but when i try to sign the applet with the new key it keeps telling me "certificate chain not found for: mykey. mykey must reference a valid keystore key entry containing a private key and corresponding public key certificate chain". this sounds like something is missing in my keystore. but it is the same that i used to sign the applet for the first time.

any ideas what i could be doing wrong?