In rest and in python sdk I can update cdrom when vm is paused.
However there is no VMCdRom.update overload that accepts 'current' parameter.

Here is java sdk code:
package java_sdk_testing;

import org.ovirt.engine.sdk.Api;
import org.ovirt.engine.sdk.decorators.VM;
import org.ovirt.engine.sdk.decorators.VMCdRom;
import org.ovirt.engine.sdk.entities.File;

public class JavaSDKTestDebug {
	private static final String URL = "https://ilia-rhevm.qa.lab.tlv.redhat.com:443/api";
	private static final Boolean debug = true; 
	private static final Boolean noHostVerifictaion = true;

	public static void main(String[] args) throws Exception {
		Api api = new Api(URL, "admin@internal", "123456", null, null, null, null, noHostVerifictaion, null, debug);
		File file = new File();
		file.setId("en_windows_7_enterprise_x64_dvd_x15-70749.iso");
		VM vm = api.getVMs().list().get(0);
		VMCdRom cdrom = vm.getCdRoms().list().get(0);
		cdrom.setFile(file);
		cdrom.update();
		api.close();
	}
}

and i'm getting next exception:
Exception in thread "main" 
code  : 400
reason: Bad Request
detail: There was an attempt to change VM values while the VM is not down. Please shut down the VM in order to modify these properties.
	at org.ovirt.engine.sdk.web.HttpProxy.execute(HttpProxy.java:118)
	at org.ovirt.engine.sdk.web.HttpProxyBroker.update(HttpProxyBroker.java:104)
	at org.ovirt.engine.sdk.decorators.VMCdRom.update(VMCdRom.java:94)
	at java_sdk_testing.JavaSDKTestDebug.main(JavaSDKTestDebug.java:20)

In engine log I can see:
2013-11-26 14:12:00,841 ERROR [org.ovirt.engine.core.utils.ObjectIdentityChecker] (ajp-/127.0.0.1:8702-12) [4a85616] Field isoPath can not be updated when status is Paused
2013-11-26 14:12:00,841 WARN  [org.ovirt.engine.core.utils.ObjectIdentityChecker] (ajp-/127.0.0.1:8702-12) [4a85616] ObjectIdentityChecker.IsUpdateValid:: Not updatable field 'isoPath' was updated
2013-11-26 14:12:00,841 WARN  [org.ovirt.engine.core.bll.UpdateVmCommand] (ajp-/127.0.0.1:8702-12) [4a85616] CanDoAction of action UpdateVm failed. Reasons:VAR__ACTION__UPDATE,VAR__TYPE__VM,VM_CANNOT_UPDATE_ILLEGAL_FIELD


I'm getting the same result with python sdk when i'm not using 'current' parameter. when I'm using it everything works fine. 
thats why we need it.

Python code:
>>> from ovirtsdk.xml import params
>>> from ovirtsdk.api import API
>>> api = API(url='https://ilia-rhevm.qa.lab.tlv.redhat.com:443', username='admin@internal', password='123456', insecure=True) 
>>> f = params.File(id="en_windows_7_enterprise_x64_dvd_x15-70749.iso")
>>> vm = api.vms.list()[0]
>>> vm.get_cdroms().list()
[<ovirtsdk.infrastructure.brokers.VMCdRom object at 0x3bc7f10>]
>>> cdrom = vm.get_cdroms().list()[0]
>>> cdrom.set_file(f)
>>> cdrom.update()
Traceback (most recent call last):
  File "<input>", line 1, in <module>
  File "/usr/lib/python2.6/site-packages/ovirtsdk/infrastructure/brokers.py", line 14009, in update
    headers={"Correlation-Id":correlation_id}
  File "/usr/lib/python2.6/site-packages/ovirtsdk/infrastructure/proxy.py", line 72, in update
    return self.request('PUT', url, body, headers)
  File "/usr/lib/python2.6/site-packages/ovirtsdk/infrastructure/proxy.py", line 112, in request
    persistent_auth=self._persistent_auth)
  File "/usr/lib/python2.6/site-packages/ovirtsdk/infrastructure/proxy.py", line 134, in __doRequest
    persistent_auth=persistent_auth
  File "/usr/lib/python2.6/site-packages/ovirtsdk/web/connection.py", line 133, in doRequest
    raise RequestError, response
RequestError: 
status: 400
reason: Bad Request
detail: There was an attempt to change VM values while the VM is not down. Please shut down the VM in order to modify these properties.
>>> cdrom.update(current=True)
<ovirtsdk.infrastructure.brokers.VMCdRom object at 0x3bbba90>