Yes, you can do that. The documentation is incomplete and does not list the mandatory arguments.

'lscm workspace replace-components <targetStream> stream <sourceStream> --all' will replace all the components in the target stream with that of the source stream. Note that if the source stream components is not found in the target stream it will be ignored. You could add the source stream components to the target stream by running 'lscm workspace add-components'.

$ lscm help workspace replace-components
Help on workspace replace-components
Replace components from a workspace. Components not found in the target workspace are ignored.

Aliases: replace-component, replace-comp, rplc

Usage: lscm workspace replace-components <workspace> <replace> <replace>
workspace - Target workspace or stream (name, alias, or UUID).
replace-type - Type of the item to replace. One of: "workspace", "stream", or "snapshot"
replace - Item to replace (must be of type specified in replace-type) name, alias or UUID.
components - Components to replace from the workspace, stream or snapshot (name, alias or UUID)

Options:
--all - Indicates all components in the target workspace will be replaced.
--certificate arg - The file containing the user's login certificate.
--smartCard - Use the connected smart card for authentication.
-b arg - Indicates the given baseline name is used to replace components.
-o - Replace components even though there are uncommitted local changes. May cause an overwrite.
-P arg - Password for repository.
-r arg - URI specifying the location of repository.
-u arg - Username for repository.

Is there a SCM subcommand to replace a component in a stream?

According to the infocenter there is a subcommand workspace replace-components that replaces components from a workspace . I guess it enables to replace one (or more components) of a workspace with the contents of the same comoponent (or components) of another workspace.
(Refer to : http://publib.boulder.ibm.com/infocenter/clmhelp/v3r0/topic/com.ibm.team.scm.doc/topics/r_scm_cli_workspace_replace.html).

Is there a subcommand that replaces the component in an stream with the contents of the same component in another stream?.

Any hint will be much appreciate,

M. Jos.