Changing from a 4/4/5 calendar to calendar month-Effects

Hi ,

We currently have the calendar starting and ending at 20th /19th of a month. The client wants to use this calendar for two yrs and later would like to change to the calendar month i.e 1st to 31st of a month. What effects would this cause to the modules and transactions.

Structure: We have two legal entities i.e UK and USA where in US has the standard calendar 1 and 31st whilst UK has the 20/19th month calendar period setup. Please let me know what would be the effects of changing the calendar if we only define the 4/4/5 calendar till 2015 and continue with the setting up the standard calendar from 2016 .