Make instant bill plan change feature the default behaviour for new accounts and configurable for existing accounts

Current behaviour

Currently when a plan is changed in the middle of a billing cycle the variable costs will be charged according to the plan subscribed at the end of the billing cycle.

Expected behaviour

Variable costs should be charged according to the plan subscribed during the the date range when any costs were incurred.

Solution

• Make "Instant Bill Plan Change" the default behavior for the following:
  • new accounts
  • accounts that opt in for "instant bill change"
  • accounts that enable billing after this change (so this behaviour will apply when they enable billing)
• Do not apply to existing accounts that have billing activated - it needs to be opt-in for existing accounts so as not to break their current billing.

Dev Notes