Can I cancel all my existing EU VAT registrations and replace them with the OSS scheme?

No, a standard VAT registration is required to use the OSS scheme. 

EU businesses still need a VAT registration in the country of their establishment. 

Non-EU businesses need a VAT registration in the EU country where they send goods from (eg. in the country where they keep their stock).

Please note that the OSS scheme is only for reporting B2C sales within the EU and no other transactions can be reported in these returns. Therefore, if you perform other transactions (eg using FBA and moving goods between warehouses in the EU), you will need standard VAT registrations in countries where you perform those transactions.