There are a couple of ways to answer this. And both should be very good (and different) news for you, though someone else may have still different thoughts. There are also a couple of points of clarification that should be made, which may not seem "as good" news, but they "are what they are" per the CF EULA (end user licensing agreement). More on that in a moment.
On the whole, though, I think what I have to say is more good news (and contrary to what you have been told) than bad.
First, perhaps "bad news" for someone wanting to get a discounted upgrade for CF10 to CF2018 is that Adobe has never supported an upgrade discount to 2 or more releases back. They have always (in recent releases) offered only a discount to upgrade from the LAST release, so CF2016 while CF2018 is the current version (which will change soon, with the public beta of the next CF release now open, when that new release comes out, and then likely only those on CF2018 will be able to get discounted upgrades to that new version).
Second, some good news there is that there IS another option to get such a discount. You don't HAVE to buy CF from Adobe. There are resellers, and one of them is Intergral (makers of the FusionReactor CF monitor), who also have a CF reselling site at buy-adobe-software.com (or more simply buy-cf.com). And at that site you will find that people running CF 10 (or 11 or 9) CAN get a 25% discount off up licensing CF2018, as part of a "25th birthday for CF" celebration.
That discount may well end when the next CF version comes out. Since Adobe has not (and never does) announce WHEN that will be, you should jump on that discount ASAP, as it seems to suit your situation perfectly.
Third, as for the assertion that you may need to buy other than a simple "one-time" CF license like before, well, that's another matter, and it seems an effort on some parts of the Adobe sales team to assert that people "running a SaaS service" via CF need to license it differently than just "buying a simple CF license". Read the EULA for yourself, to see if you would be in violating of it (such as if running a "service bureau", etc). There have been others from Adobe who have piped in (here and other public places) to say that those sales people are over-stepping in their accusations.
Bottom line, if you are not in violation of that EULA, then it would seem you need not buy more than a simple "one-time" license like before. Of course, each client (and lawyers and judges/arbitrators) needs to make the final call for themselves.
Finally, as for how many cores you have, that too is indeed covered in that EULA, and it may be "bad news" for some. For CF Standard, it's licensed "per 2 cores", so if you have CF on a 4-core machine, you are supposed to buy 2 Std licenses for that one machine. If it's an 8-core machine, then you'd need 4 (which is more expensive than a single CF Enterprise license, which itself covers 8 cores--so for some people, it's cheaper and better to go to Enterprise. I did a talk recently on the differences and those matters. More at the presentations section of carehart.org).
(Adobe is not the only company to charge "per core". Heck, SQL Server is priced per core and can quickly become astronomically more expensive than CF on a same-configured box.)
So I hope that helps you, and that you see it mostly as good news. I know that some see any discussion of having to "pay for CF" as one more reason to deride it and people who do pay for it, and see it as another "nail in its coffin". But for some, it's totally worth it to pay for CF, and I hope we can avoid others coming on to make this yet another debate about that topic.
But what you've raised is an important one, that people do share news of once in a while. It seems rather few and far between, which makes me think it's some segment of the Adobe sales team that engages in that ploy. trying to drive people to a different annual license. Let's see if anyone else has anything more (especially more authoritative) to say.
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