Example: A patron joins your Patreon community on January 5th for $10 per month. The patron is charged on February 1st for January patronage. On February 25th, you change your payment type so that patrons are charged up front. The patron will be charged $10 on March 1st for March patronage. If this patron increased their pledge on February 26th to $20, they would be billed $20 on the 26th for February.
On the 1st of the month (and every 1st of the month thereafter) they will be charged the full amount of their pledge until they cancel or edit their pledge.
After your first month of charging patrons up front, when a patron increases their membership, they'll immediately be charged the difference between their recent payment and new membership value.