Typical bluegrass lawns should be watered about once a week in spring and fall and up to 3 times a week in July when temperatures are high and day length is long.  And before you change lawns, think about your trees.  We are seeing many trees tank when the landscape is majorly changed.  The root systems are a minimum width as twice the height of the tree.  So a 50' tall tree has a 100' wide root system.

Once buffalograss is establish, you can water as little as once a month.  Depends on how green you want to keep it.  Our Fruita research station has had success with adding blue grama with the buffalograss and waters about once a week to keep it greener.  I have not overseeded buffalograss over bluegrass.  Most seed needs light to germinate.  Our statewide turf specialist has put together how to establish buffalograss: https://extension.colostate.edu/topic-areas/yard-garden/buffalograss-lawns-7-224/

Sincerely,

Susan Carter, Horticulture agent

Yes, if you are going to change the watering over the trees, you need to do it slowly over a period of months.  They do not like change, especially quick change.  

Here is our lawn care document.  Sometimes even dialing in the care of the existing lawn can save money, and time. https://extension.colostate.edu/topic-areas/yard-garden/lawn-care-7-202/

Thank you for using the Extension office.

Susan