pandas dataframe - set column values depeding on condition

I would like to set each element of 'col2' to ('col2'+ 1) if ('col2' < 'col1').

Unfortunately everything I've tried fails.

Example code:

import pandas as pd
df = pd.DataFrame({
    'col1': [2, 1, 9, 8, 7, 4],
    'col2': [0, 1, 9, 4, 2, 3]
})

print(df)

#   col1  col2
#0     2     0
#1     1     1
#2     9     9
#3     8     4
#4     7     2

df[(df['col2'] < df['col1'])] helps to select the relevant rows:

#   col1  col2
#0     2     0
#3     8     4
#4     7     2
#5     4     3

but as result I need:

#   col1  col2
#0     2     1
#1     1     1
#2     9     9
#3     8     5
#4     7     3