How to properly change plan?

[FrancescoMussi]

I have two plans: monthly-plan and yearly-plan. But of which have a trial period of 30 days.

I need to let the user switch plan.

First question: Is this the proper way to change plan: Subscription::where('billable_id', $user->id)->first()->setPlanAttribute($new_plan_id);

Second question: If I subscribe to another plan I will get the 30 days free trial. But what if every 29 days I keep switching plans and every time get the 30 days free trial? How can be avoided this?

Thanks