How can we improve WHMCS?

I was thinking about modifying how WHMCS calculates the price for upgrades and downgrades as it's currently being done incorrectly (https://requests.whmcs.com/idea/change-how-upgradedowngrade-amount-works-when-changing-billing-cycle).

We currently have the following parameters to work with:
Array
(
[oldproductid] => 1
[oldproductname] => Pro V
[newproductid] => 1
[newproductname] => Pro V
[daysuntilrenewal] => 365
[totaldays] => 365
[newproductbillingcycle] => monthly
[price] => 277.42
[discount] => 0
[promoqualifies] =>
)

For my fix to work, I would need to have "oldproductbillingcycle", "oldprice" and generally stuff related to the current service - but there's currently no way of accessing that.