'dismantling a bridge' can mean someone distroyed it completely but 'dismantling a house' does not mean so because of the phrase 'before knocking it down.'
Though 'dismantle' and 'disassemble' are a bit opposite they have a number of similarities and are sometimes(very few) used in place of each other.
Now I see what the phrase "race upstairs" menas. Your answer confirmed my doubt. I got this phrase in a bilingual dictionary, which gave its meaning as that someone chased another to get upstairs. I was skeptical of that explanation, and then came to ask for help.