In principle, a foreigner who has a visa waiver (B1, B2) or short term visa (C1, C3, C4) cannot change the visa while staying in Korea.
They must depart and apply for a new visa at the Korean consulate overseas. However, there are some exceptions as below. Contact me for more details.
- Dependent family (F3)
- Overseas Korean (F4)
- Some subtypes are not available.
- Nationals of Ukraine, Uzbekistan, Kazakhstan, Kyrgyzstan, Tajikistan, Russia and Turkmenistan are not available.
- G1-10 visa holders who need the medical treatment
- Germans who enter Korea with B1 can change to another visa except D3, E9, H1.
- Canadians who enter Korea with B2 can change to D1, D6, F1, F3, G1 for 6 months.
Even if it is available for some cases like before, you must prepare required documents for changing visa status within your period of stay which is usually 1 month to 3 months. So your time can be running out before filing an application to the immigration office. Moreover, those are just exceptions and it is possible to be rejected for some unpredictable reasons.
So I recommend not to enter Korea with a B and C visa deliberately to change to a long term visa type for whatever reasons. You should apply for that visa in the Korean consulate.