Bear in mind that some countries (i.e. UK) will not allow you to change visa categories while already in the the country. You must leave and apply for a new visa from your home country.
Example: in the UK if you hold a valid student visa (Tier 2) and find a company willing to employ you based on your expertise (Tier 2), you must to go back to Brazil and apply for a visa "Tier 2" from there and then come back to the UK.
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Bear in mind that some countries (i.e. UK) will not allow you to change visa categories while already in the the country. You must leave and apply for a new visa from your home country.
Example: in the UK if you hold a valid student visa (Tier 2) and find a company willing to employ you based on your expertise (Tier 2), you must to go back to Brazil and apply for a visa "Tier 2" from there and then come back to the UK.
Edit: student visa (Tier 2)
student visa (Tier 4)
It's the same in US.