How to convert internal activity to service activity

Hi,

In a project - network, while creating a service activity by mistake an internal activity got created (with control key PS01). Now I want to change the internal activity to service activity. I can change the control key from PS01 to PS05. But the "Service Exists" indicator which is to be tick marked is not getting activated due to which I am unable to change the internal activity to service activity.

Pl. suggest how to do the above change.

With Thanks and Regards

Ratna Palit