What is the predictable behavior of changing SOL_SOCKET, SO_RCVBUF on the fly on a UDP socket?

What should be expected to happen if we resize the input buffer of a UDP server socket on the fly on a Linux system?

setsockopt(sock, SOL_SOCKET, SO_RCVBUF, ...)

I am particularly interested in these questions:

• If I shrink below what is currently in the buffer, would this simply drop the oldest/newest? datagrams properly, or could it flush everything that's there, or worse could it corrupt data such as truncating a datagram?
• Would shrinking the buffer even save memory or something prevents that memory from being reused by the system?
• Is the behavior predictable or could it behave randomly at times?