Já temos uma pergunta extremamente semelhante sobre como alternar de um visto C para um visto D de longa duração . O consenso é que esses tipos de interruptores não são regulados em nenhum lugar, então não há razão para que os funcionários da fronteira neguem sua entrada.
Experiência de pessoas em vários fóruns também confirma:
My country doesn't need a visa to enter the EU/Switzerland, but I did need one to be a student. I took a leap of faith and came in exactly 14 days before it was valid, because I was told I need to register with the Kreisburo within 14 days. I am still not sure this is the exact number of days, but I believe it was at the time. I phoned the Swiss Embassy in my country before and they said FOR CITIZENS OF MY COUNTRY there should be no problem. I asked them for some kind of proof, article to support my claim but they didn't have anything. I wrote down the number of the Embassy who told me this, in case I ever needed it.
My port of entry was Amsterdam. The guy looked at me and said "Your visa is not yet valid..." I said I knew. He talked to his colleague and they agreed I was allowed to enter Europe as a citizen of my country regardless of this student visa business. I honestly think they could have said no just as easily but they were probably having a good day or something.
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I had a similar experience as avita, I arrived to CH about 4 days before the start date on my visa since the people at the Swiss consulate told me it would not be a problem since citizens of my country don't need visas for short time stays. The Federal migrations office confirmed that. So, I arrived 4 days earlier and registered two days before the start date of my visa. The lady at the registration office didn't bat an eyelash, everything was smooth.
Veja também nossa pergunta relacionada ao passar de um Visto de Residência para um Visto de Turista , no caso de precisar de ficar mais tempo no final da sua visita a Portugal.