We take both Full Faith and Credit, and Privileges and Immunities so much for granted, the only way I think I can explain it is with examples. Read them. I'm sure your reaction is going to be well yes of course. 1787 was a different world.
It is no surprise that future new states were anticipated. It does surprise me how little is said about it. This being a constitution, it goes without saying that anything not specified is left up to the Congress. The actual process set up by Congress starts with an enabling act from Congress that authorizes the potential state to start the process. The process then involves setting up a draft constitution for the state, after which they apply to Congress for admission, and if they are approved by a vote of Congress, they are in. All states have the same footing, new and existing.
The one thing that does surprise me about this subject is that Congress retains the power to sell territories
The final subject with regard to the states is that they are guaranteed by the federal government to have a republican form of government, be protected against invasion, and protected against domestic violence. This is the guarantee to have the federal government nationalize whatever portion of states guards are deemed necessary for the state in trouble. NOTE that the state legislature is the one who gets to make this call, and the federal government has guaranteed to respond. It is NOT up to the President to either make the call or decline to respond. If either of those two are chosen, the act involved is unconstitutional.