[Plugin: s2member pro] Pro-Rate membership on upgrade

I am looking for help to pro-rate an existing users subscription when they upgrade their membership.

For example: a user purchases a basic yearly membership in January and then in June decides to upgrade with a custom capability (also on a yearly subscription). Out of the box, s2Member cancels the previous (existing) subscription and then creates a new one, charging the user the full rate for both subscriptions. I will have some very upset customers if I charge them a full rate in that instance!

It seems to me the easiest way to go about this is to calculate the refund amount for the first subscription and then apply that amount as a discount for the upgrade. This method would set a new EOT of 1 year from the date of upgrade.

However, I would prefer to continue the original EOT rather than start anew. So this would then mean figuring out the percentage of time that is left and applying that to the amount.

s2Member support tells me that this will need to be done with PHP and accessing various s2Member PHP Constants (WordPress Dashboard → s2Member → API / Scripting → s2Member PHP/API Constants).

They pointed me to this article explaining how this process works in principle:

Specifying a future start date for a recurring subscription (aka Custom EOT)

I am not skilled for this type of coding and am wondering if anyone out there can help? I didn’t realize this wasn’t built into the core of s2member pro and am quite surprised as it seems like quite a basic need.

Thanks!