Can I exchange the positive signal (p) and negative (n) signal of LVDS?

Yes.

Of course, you need to negate the signal inside your design.

Important notice: you can't put the inversion between a DDRIO (or ALTLVDS primitive, which uses DDRIO) and the pin.

So, if you're planing to use DDRIO or ALTLVDS, beware of that inversion.