Read this document
for a discussion of AES and DES encryption and why AES is superior.
Normally servers and clients negotiate the encryption protocol to use by sending a list of understood protocols to the other system. The server and client will use the highest protocol that they both understand. If one understands AES and DES but the other only understands DES then they will settle on DES encryption. The only way to force AES encryption is to change "server A" to reject DES or lower encryption standards. This will work, but then "server B" will not be able to establish a secure communication channel with "server A" until the encryption software on "server B" is updated.
I don't know how to disable encryption protocols below AES, perhaps someone else could help there. If "server B" is upgraded to use AES encryption but "server A" remains unchanged, then another server could possibly connect to "server A" with DES encryption. If DES is not secure enough for your communications between "A" and "B" then I would assume that it would be undesirable for any other machine to communicate to "A" at a protocol below AES.
If you get nothing else from my post, you have a good link to a document that explains AES and DES encryption and why DES is not a good choice for sensitive information.