Client upgrade loses initial expiring date

Hi,

when using Pro Sites (Versión 3.0.3),

after a client upgrades from plan A 1 month to plan A 3 month,

it looses the original plan expiring date:

Ex:

plan A starts Jan, 1st. for a month (finishes 1st. February)

then client wants to upgrade for a 3 month plan:

plan A should then finish May, 1st.

instead, plan A monthly is removed, and starts a 3 moths plan on jan, 1st. expiring on April, 1st.

(client is losing 1 month subscription which was already paid).

Is it a bug or a configuration error?

Many thanks,

Carles