I can't predict how upgrades will work in the future, but I think you've been able to skip versions in the past, possibly with a smaller discount.
With recent upgrades there has been a time element to upgrade discounts, so that if you bought Opus very soon before a new major version came out, you got a much bigger discount, or even a free upgrade. But I don't know if the discount was lesser if you skipped a version, and it's probably all subject to change anyway since nothing about Opus 13 pricing has been decided yet.
For a definitive answer, I would email sales (see the first part of the Licencing FAQ for the address).
What I can tell you is that Opus 13 is not around the corner. We are still working full-time on free updates to Opus 12, and we have not even started on 13 yet. If you're waiting until Opus 13 to upgrade, you'll be waiting for some time.
12.8 does not mean Opus 12 is 80% over. There'll be a 12.9, a 12.10, a 12.11, etc. The 8 is just a number.