Someone more specifically trained and experienced than me is welcome to correct me, however I have found the following solution is quite simple and has never broken me:

1. Copy the GUID {WITH curly braces}

2. Alt+V, C - opens the XML text

3. Ctrl+H

3a. Scope your "Look In" to "ENTIRE SOLUTION"

3b. Paste your GUID to the "Find" box

3c. Make sure your "Replace" box is empty

4. Replace ALL

This will force SSIS to re-identify all connection managers using that stale old GUID with a fresh new GUID!