Well I thought I was going to have to tell you that there wasn't a way to restore the link, but then I remembered a features that was in 4.1 (removed from 4.2 due to changes with the prompting model and source code size concerns).
Here's what you do. Open your existing stored process in your existing project. On the "SAS Code" click the "Include code for" button then make sure that the "Stored process definition" option is checked (if not check it, by selecting it). Now on the bottom left of the Stored Process Manager panel press the view code icon. Select all the code and copy it. Near the bottom this should have a section that looks something like this:
*
* tVVbb5swFP4rFi/dHgiXkNtEkDKStNHWNYK2T5UmB04ia4CZbbrk38/mUiBNo70sDwnn9n
* 2fjw8nbigog3jLaASce+4dyYT8uQeBYyxwADnlRKacNrG3MMuPNViMgpXl+HeucTHP/U4P
* JMJJCOwVmBcuwntMMqSj2o8qTlSToirPNfplfZTbp83Ss0azobUaWvp4PFnrlrUc6dOp6e
...
* /ZXw==
*
Paste this code into a simple editor of your choice (MS notepad for example). Now close the project without saving anything.
Open a new empty EG project and select File->New->Code to get an empty code window. Select the text you placed in the editor above and paste it into the code window. Return to the process flow, right click the code icon and select "Create Stored Process". This wizard should have all the information (including parameters) from the previous stored process. I recommend that you save this stored process and it's source code under a different name, until you are happy with the operation of the new stored process and it's relationship with the copied code.