A flat sea-level half does not predict the same pace in a mountain town. Thinner air raises heart rate at the same effort. The honest adjustment is to start slower, not to hope the first mile feels normal.
What changes
Above about 5,000 feet, most runners notice breathing earlier than the pace chart suggests. Effort is the useful signal. A watch pace copied from home will usually feel too hard by mile two.
Arrival does not equal acclimation
Landing the night before is fine for one race. It does not acclimate you in 12 hours. A longer stay can help some runners, and it can also leave others flat. For a single destination race, plan the pace for the altitude you will race, not for a week of adaptation you will not get.
Drink more than you would at home. Alcohol the night before hits harder at altitude.
How to pace it
Start a few seconds per mile slower than the sea-level goal. Hold effort on climbs. Do not try to bank time on the first downhill. If the race is a Boston qualifier attempt, read the course elevation before you treat a flat-course equivalent as the target.