Plan expiration date update using script

Hello,

I have the following scenario:

About 400 user wth two active plan:
Plan A: 1 year
Plan B: 6 month

Plan B was activated after a variable period from Plan A activation.

I need to extend Plan B in order to let expire both plan in same time.

What I want to do is tu update using a script the table payplan_subscription, field expiration_date.
I will set expiration_date of Plan B same as expiration_date of Plan A for the same user.

Is this enough or I'm missing someting?
Is ther another way? Maybe an app?

Thanks,
Mario