Is a 1024-bit DSA key considered safe? I created my PGP key in 2000. I’ve revoked the older, weaker sub-keys in favor of a 4096-bit RSA one, but the primary key is 1024-bit DSA. I read on Wikipedia that… 


  NIST 800-57 recommends lengths of 2048 for keys with security lifetimes extending beyond 2010.



Should I consider my primary key insecure?
If so, is there any way to replace it without revoking my entire key and creating a new one?
If my primary key were compromised, the attacker would be able to create signatures as me, but they wouldn’t be able to decrypt information that was encrypted to my public key, right?
"Secure" is not a binary, black-and-white thing.  Instead, it's about risk management.  Instead of asking whether something is secure, it's better to ask whether it is "secure enough for such-and-such purpose". On the one hand, 1024-bit keys are uncomfortably close to what can be cracked, given lots of computational resources.  On the other hand, for casual use, it's probably fine, and there's no need to go through a painful exercise to replace your key.  But if you're generating new keys, these days it'd be a good call to use a 2048 bit key.

On your third question, someone who recovers your 1024-bit private signing key today would not be able to decrypt past ciphertexts, but they would be able to mount man-in-the-middle attacks on you in the future (which would let them decrypt information that anyone encrypts to you in the future), assuming they are willing to mount active attacks.