If you are using the standard form for printing POs, this is possible. Have look at the config spro->MM->Purchasing->Messages->Fields relevant to printout of changes.
Here add the table and field for which you want to show a text in the printout saying 'X field has been changed'.
This 'text' is again configuration and can be added from spro->MM->Purchasing->Messages->Texts for messages-> Define change texts
In case the standard way of determining the text does not suit your requirements, you can also determine the text using a routine and assigning to the 'printout of changes' config.