Select whitespace from first line only using regex in python

I would like to match and substitute whitespace that appears in the first line of a CSV.

For example I want to substitute whitespace from the first line only with '_':

"product id","Region","Region Code" 
"888","North America","GEO123"

To give:

"product_id","Region","Region_Code" 
"888","North America","GEO123"

This is my current approach:

f1 = open('file1', 'r')
f2 = open('newfile', 'w')

for line in f1:
  f2.write(re.sub('([\s])+', '_', line))
f1.close()
f2.close()

Which replaces all whitespace throughout the document. How can I adapt this so that it only works on the first line of text?