Ok, just called ICON.
If you deferred last years pass, you are still considered a "renewal" rate and Not a "new" pass holder. You cant use last years credit until Apr 19/21 though.
So if my logic is correct and i paid 600 last year and my renewal rate is 650 for the plan I choose then i apply my 600 and i owe them 50. If i use my credit and pay the 50 on Apr 19 then i can ski on Apr. 19 at the resorts that allow it like Mammoth.
Hope i heard that correctly
It is now written on their website FAQs