Android application certificate validity

Question

Is it possible to change the validity of a certificate?

key-a:has validity of 2 years.
apk-a:signed with key-a.
apk-b:signed with ? which has 25 years validity.
apk-a and apk-b are the same applications with the same package name.

Suppose there is a user who has installed apk-a on his cell phone.How can i make it possible for him to install apk-b as an update.
Here what i have tried so far:
1.)I tried using keytool the expand the validity of the existing key with the below command.
keytool -selfcert -v -alias mycertalias -validity 80000 -keystore teststore.jks -storepass mypassword
result: apk-b which is signed with this key cannot be installed on the cell phone.(requires previous one to be uninstalled.)

2.)I have created key-b which has a validity of 25 years. and signed the apk-b with both keys.
result:same as above.

ps 1: I have tested these on samsung galaxy i7500.(which has android 1.6 installed on it)
ps 2: My application is on a website.I got nothing to do with android market.

Answer 1

Suppose there is a user who has installed apk-a on his cell phone.How can i make it possible for him to install apk-b as an update.

That is not possible, sorry. They will need to uninstall the old application and install the new one.