Is there a way to keep my number and switch to the micro?

I bought a galaxy s4 active from an individual she told me it is unlocked and would work with my sims if it didnt that i can get a sims card free of charge that will work. I  do not want to change my phone number and i just bought straight talk time the sims i currently use is a nano but galaxy takes a micro is there a way to keep my number and switch to the micro?