I took one of the three VIRGIN bundles (100,150, 215) and I changed to another bundle (e.g. VIRGIN 70), do I get the OSN+ add-on?

No, customers who change to any other package (e.g. VIRGIN 70, Virgin 45), will not get OSN+ add-on. If the customers went back to one of the three VIRGIN predefined packages (100, 150, 215) and still on the validity period, the customer will resume his OSN+ add-on until the expiry date.