What issues arise where landlords agree to vary to monthly rents?

Most leases require tenants to pay rent quarterly in advance on the four quarter days in each year. However, during the credit crunch continues to bite, the British Retail Consortium has called upon commercial landlords to change the frequency of rental payments and to accept rents monthly in advance.

See http://www.brc.org.uk/rentmonthly/default.asp?content_id=1

What are the issues that the parties should consider when documenting any temporary concession? See below.