Upgrading members to an annual plan

Hi

Some users join our site on the monthly plan, but then want to upgrade to the annual plan.

currently we tell them to do this:

  1. log in
  2. cancel monthly membership.
  3. while logged in go to the “join” page and click on the annual plan.

is there an easier way to do this, one that possibly works out what we owe them for the remainder of the current month?

thanks