It is sometimes assumed that because PGP supports older cryptographic methods that it must be at risk of something called a downgrade attack. The basic idea is that if you can't break a particular type of encryption then you trick the system into switching to something you can break. That might be no encryption at all.

When PGP is used in a unidirectional application like email a downgrade attack is impossible for the simple reason that any sort of negotiation of the method to be used is impossible. Negotiation would require some sort of reverse channel which in this case doesn't exist.

Any preference information is embedded in the PGP identity (public key). This is the preference information from an identity generated from a recent version of GnuPG:

[ultimate] (1). Password Store <pass@store>
     Cipher: AES256, AES192, AES, 3DES
     Digest: SHA512, SHA384, SHA256, SHA224, SHA1
     Compression: ZLIB, BZIP2, ZIP, Uncompressed
     Features: MDC, Keyserver no-modify

The preference information is in the secure part of the identity. That means that it is signed by the public key also embedded in the identity. This is the public key that will be used to encrypt the email so the preferences are linked to the public key in a way that means that you would have to break the underlying encryption to modify the preferences. If you could do that then you would not have to bother with a downgrade attack.

Ultimately this all works because the type of public key cryptography (e.g. RSA, ECDH, ECDSA) is entirely determined by the type of public key in the identity. If you don't have the ability to generate your message using that particular type of public key cryptography then you can't send a message to the owner of that identity. There is no way to request a different method short of asking your correspondent to generate another identity.

PGP means that you get to choose how you want your messages sent to you. There is no opaque process somewhere in the middle that can override that preference.

PGP FAN index