Hi Anna,

These are great questions.  In general, yes Blue Grama and Buffalograss grow well just about anywhere in the state, including Highlands Ranch.  While you don't need to physically remove the existing lawn, you do need to kill it and then rake away as much debris as possible before preparing the area for overseeding or sodding.  You can use something like a rototiller to prepare the area, or even use a core aerator and heavily aerate the area before seeding (aim to have the holes on 2" centers).  

This CSU resource on buffalograss lawns covers the basics; the instructions remain the same if you are doing blue grama or a mixture of both types.  Note that in the first year during establishment, these grasses will require ample water.  You will see water savings beginning next year!

https://extension.colostate.edu/resource/buffalograss-lawns/

I hope this is helpful.

Cordially,

John