There likely is a text label that specifically is coded as 'GST' on your form. You could make a copy of that form and save it as a new form, and edit the label GST to HST. Now there are 2 forms, one for GST and one for HST.
Alternatively, depending on which application and version you are running, you could insert a database field that prints the tax name, rather than a field label. Thus you would only need one form and it would print GST or HST depending on the Tax Group assigned to the invoice.
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Softrak Tech Support