While this feature is due to be released in the near future, here is a way to do this by connecting to the OfficeClip sql database.

Run the following sql command:

update userpayrolls set timesheet_period = {0} where user_id in {1}

where {0} is one of the following:

1 = Weekly
2 = Bi-Weekly
3 = Semi-Monthly
4 = Monthly

{1} is the comma separated list of all the user ids (found from the users table). If you want to make changes for all the users in the system, just ignore the where clause.