No.
Membership is on a rolling/stack basis.
What that means is if you bought your €20 Don Membership today, it still won't kick in until your Current one expires...
Unless you use a serious amount of Data, is €3 for 1G Data in the interim a good bridge until your Membership starts?
P's if you are going out of the Country, you may consider omitting the purchase of Data altogether, as 48 do not support Data Roaming currently.
I hope this answers your question.