Invoice format has been uploaded here: https://help.gstplus.com/hc/en-us/articles/212483285

There is NO concept of Central GST Registration and State Registration. Only one registration per state would suffice.

Invoice Format can be referred from below link :

https://help.gstplus.com/hc/en-us/article_attachments/206940789/5_draft-invoice-rules.pdf

Dealers already registered under VAT, will be automatically moved to GST and issued new GST Numbers.

When should we apply for GST. I am already a registered VAT dealer.

Every person who is liable to be registered under Schedule III of the Act shall apply for registration, within 30 days from the date on which he becomes liable for registration as per the prescribed rules.. Registration is required if aggregate turnover in a financial year exceeds Rs.9,00,000. This threshold limit will be Rs.4,00,000 if the taxable person conducts his business in any of the North Eastern States including Sikkim.

Existing registered taxpayers would be migrated to new GST network. This will be on provisional basis for 6 months. All such taxpayers will have to fill up form GST REG 20, online to finalize the registration/enrollment process. On completion of this GSTIn would be issued.