From TECH BLOG FOR EJBCA ENTERPRISE OPEN SOURCE PKI's articel Converting keystores between JKS and P12.
Johan dug up these keytool commands, that with in JDK6 and onwards (not in JDK5 and earlier).
JKS → P12
keytool -importkeystore -srckeystore keystore.jks -srcstoretype JKS -deststoretype PKCS12 -destkeystore keystore.p12
P12 → JKS
keytool -importkeystore -srckeystore keystore.p12 -srcstoretype PKCS12 -deststoretype JKS -destkeystore keystore.jks