It is still on the list:
https://developer.paypal.com/api/nvp-soap/ipn/IPNandPDTVariables/
and I think they are distinctly different. I think the refund has its own transaction ID, but the Parent Transaction ID is the id of the original purchase.
I would say don't change it. Most likely the person that told you that didn't know what you were doing. I'm not positive... I'd like to know how and why Paypal told you that, but it seems wrong to me.