If you have submitted your declaration to Revenue, then you an continue to make any changes you wish. Once you submit your declaration, you should not make any more changes until you have received a response back from Revenue --either a "returned with erros" or "MRN allocated".
Once you have received a response or received an MRN, you can amend only a limited number of items on your declaration such as:
Flight no etc.
If you make these changes, you will need to resubmit your declaration. You will get the same MRN back with updated shipping details.
If you have received an MRN, you are not permitted to change or amend:
Consignor or Consignee
Any product Detaila
Country Of Origin on your PML as the TARIC rules that get applied are based on the country of origin. If you want to change COO you will have to add another product with a different COO
If you want to change the above you will have to cancel the declaration and start over.