So I've been admitted to master's program in Mechanical Engineering at a US university. 

I contacted 2 of the faculty in the department in my chief area of interest , but neither had an opening/ funding . I feel like it was a huge blunder on my part. Should I try to find an advisor in a different subfield or wait a semester and see if any new opening arrive?

If I start research in a different subfield and a spot opens up in my preferred subfield next semester,  is it possible to switch?