re: New Job Changed Schedule on First Day...Any Recourse?

If it were me I would probaly politely show my boss my offer letter and explain I accepted the offer as it was presented in the letter.

And then I would shut up and let him explain what happened between the time the offer letter was prepared and when I started work. I would avoid letting my anger & disappointment show as much as possible.

If the explanation sounds plausible, I would grin and bear it.

If it doesn't, I would look for another job.

If you don't trust your company that it is good for its word, you will never be happy there.