Sir,
As an unregistered dealer, you do not need any registration or change in your existing registration.
However, if you want to issue Cenvatable invoices, you must obtain a separate registration as a dealer. In any case, you have to make sure that your manufactured goods and trading goods do not get mixed up.
Yes, in the name and address of the same Factory Premises, you can hold two Excise licence number, first one as a Manufacturer and another as trader/dealer.
You can apply for dealer registration at the same factory premises address and can get dealer registration thereon; subject to certain conditions.
Though the registration number shall be different from manufacturer but there shall be certain similarities in both the numbers, however you need not to get confuse with such registration numbers allotted to you.