There's nothing unique about America that would prevent this from happening.
If a state were to change their constitution and institute a state police force as the primary and singular police agency for that state, the obvious answer to your post would be that all the cops in the state would get rolled over to the new state agency.
You're making a lot of noise about a national police force, but nobody else really is....and there's not even a legal infrastructure to support the idea. It would require a constitutional amendment so substantial that it would in effect be a completely new constitution.
There will be no national police force. You're ok. The sun will rise tomorrow.