I guess they'll have to get a new number if that happens. Interstate 59 already exists, running from Louisiana to Georgia. I'm kind of curious what that process looks like.
Google maps already has it as Interstate 69 through Houston, and transitioning back to a state route north and south of the city. The actual numbering system is very simple, and explained at https://www.youtube.com/watch?v=8Fn_30AD7Pk
https://en.m.wikipedia.org/wiki/Interstate_59