Currently, we allow altering of types. However, because we no longer store the length for all types anymore, switching from a fixed-width to variable-width type causes issues. commitlog playback breaking startup, queries currently in flight getting back bad results, and special casing required to handle the changes. In addition, this would solve CASSANDRA-10309, as there is no possibility of the types changing while an SSTableReader is open.
For fixed-length, compatible types, the alter also doesn't add much over a cast, so users could use that in order to retrieve the altered type.