If I'm understanding what you need to do correctly, you can easily do it using the datetime package.
First, convert your variable to a date:
import datetime
date = 20200930
dt = datetime.datetime.strptime(str(date), '%Y%m%d')
Now, when you print this, you will see:
datetime.datetime(2020, 9, 30, 0, 0)
.
You can then add a day to it:
dt_new = dt + datetime.timedelta(1)
And specify the format you want to see the new date variable in:
print ('{date: %Y%m%d}'.format(date=dt_new))
which will give:
20201001
.