Thanks for providing us with details.
There are two options:
1. When your users want to switch from free to paid plan in the middle of their free term, you could update their subscription using "Change Subscription" option (available in the subscriptions page on the right hand side), change the plan and while applying changes, remember to select "On Next Renewal". When this is updated, charges $5 will be applied only during renewal, however, no immediate charges will be processed. For processing the payment immediately, you could "Change Next Billing Date" to 'Now' that is available on the right hand side. Here's a video on this.
2. You could have different billing cycles for the free plan (weekly, 4 weeks) and paid plan (monthly, 1 month) and when you change the plan in the middle of free term, the subscription's billing cycle will be automatically reset (because of different billing terms) and customer will be charged immediately when you apply the changes "Immediately" with proration disabled. Here's a video on this.
Would any of these work for you?
We have a B2B SaaS
Every new company that registers our service starts with a free plan.
In order to have valid KPIs (Free vs Paid plan customers) we transfer all registered companies to Chargebee. In Chargebee the customers get a "Free" plan. We have activated Proratio.
When a company switches from Free to a Paid Plan (for example $5/month), then the customer gets charged not with $5, but with proratio-reduced price (for example $2,50 if the company upgrades 15 days after registration). This doesn't fit for us. First time charge of a paid plan has always to be the full price.
Is there a possibility to charge fully prices of $5 when switching from "Active" Free Plan to "Active" Paid Plan?