RE: Change all the VAT to GST
Depending on a certain setup, you might be successful, but you should understand in details what exactly result you need to get. This relates to the transactions that will be calculated in the system. Apart from that, you would need to have your reporting - this would be either manual work or additional setup in the system using Account Schedules and exports to Excel or other electronic format, depending on your local requirements.
As Suresh mentioned, you can opt for the localization where it has been already implemented by default, e.g. Australia or New Zealand.