Best approach is to not use a value in your Purchase Order record that is entered by hand. This approach leaves you vulnerable to this complication. Instead, define an auto-entered serial number that you use to link a PO record to its line item recprds. Hide this field, you don't need to put it on any layout where it can be edited. Keep your hand entered PO Number so that you can print it out on your layouts, just don't use it in the relationship.
If you insist on using a hand entered value as your primary key, first put a unique values validation rule on it so you can at least catch errors where the PO number duplicates a value in an existing record. Then, to change the value without loosing the related records, you must first find those related records on a layout where you have their PO Number field exposed for editing, you change them to the new PO number. When you return to your PO layout, the line items have been "divorced" from your current PO. Update the PO Number field for the PO record and they will reappear. Fail to do this exactly right, and you'll either get records linked that shouldn't be linked--possibly mixed in with those that should or you may still find that they are all missing--hence my advice to avoid the problem in the first place.
Ah, ok gotchya, yea, thats probably best to make a unique serial number field - I'll do that. Thanks again!