No, you cannot convert a Singapore Visit Visa into a work or student visa while in Singapore. To work or study in Singapore, apply for a long-term visa like an Employment Pass or Student Pass before you arrive. Your employer or school will help you with this. Trying to change a tourist visa to a work or study visa, or staying longer than allowed, breaks immigration laws and could result in deportation or being banned from re-entry. Make sure you apply for the right type of visa beforehand.