Plant XXXX not assigned to XXXX company code of Network

My issue is we have multiple plants within a project that belong to different company codes. Initially the project is created with a single plant and later on determined to produce certain products from another plant. When changing the plant , it appears that if shipping network is already created , it greys out the plant/company code and hence throws an error 'Plant XXXX not assigned to XXXX company code of network'.

Are there rules regarding when a plant can be changed and when they cant. I am new to PS and so not very knowledgeable on situation. I see from other postings that it is possible to change plants in a project. However what are the conditions under which they can be changed. If someone can explain , would appreciate it.

Thanks

Rama