Use SHA512 as encryption in Multicraft panel (which you can change the settings for MD5), but I need to use an older version of the same database. This old version did not have the option to encrypt with SHA512, but only with MD5. Thus, all passwords are invalid with MD5.
It's possible convert all SHA512 passwords in MySQL database to MD5?
SHA512 and MD5 are hashes, not encryption algorithms. By design, they are not reversible.
The only way to convert these values is to wait for each user to log in, validate their password against the existing SHA512 hash, and rehash¹ their input with MD5. This is the reverse of how password hashes are updated to more secure standards.
But please, please, don't do this. MD5 is hopelessly broken. You would be doing your users a huge disservice to revert from SHA512 to MD5. Find a way to use the newer version of your software.
¹As noted by zaph in a comment, "rehashing" is an oversimplification, and depending on how your panel is actually implemented it might be using insecure password storage today.
To provide reasonable security each password must also have a unique random salt (which protects against things like rainbow tables) and each hash must be iterated enough times to make brute forcing impractical. As computers get more powerful the number of iterations must be increased. Today it is common to iterate tens or hundreds of thousands of times.
Cryptography is shockingly difficult to get right. Instead of trying to follow all the best practices manually, use libraries and functions that operate at the right level of abstraction and have been audited for security. An algorithm like bcrypt (via PHP's built-in
password_hash function, where it is currently the default algorithm) would be a good choice.