If a custom record is already created, can i assign a parent record? I would like to have this custom record pull from the transaction side as well. As of now the child record has transaction as a parent record but the parent record itself does not have

If I understood correctly what you would like to do, you have two custom records, parent and child. The child have a relationship with transactions and they are used as parent. Your need is that both the child and the parent custom record are related to the transactions record using them as a parent. I suppose that you already have in the child custom record the relationship with the parent custom record as a parent, right? If so, why you need to have a relationship at child level? Why not adding directly the transaction as a parent of the parent custom record (simply adding a field that relate to the transaction and flagging the checkbox “record is parent”)? In that way, if the child custom record has the father as parent, then in a relationship model it will inherit the relationship with the transaction that is parent of the parent custom record 🙂