How to upgrade CentOS to RHEL without Subscription?
Hello,
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For converting CentOS systems to RHEL, Red Hat make available a utility called "Convert2RHEL". Now, I've never used this myself, but Red Hat have some fairly extensive documentation on it at the following links:
Convert2RHEL: How to convert from CentOS Linux and Oracle Linux to Red Hat Enterprise Linux
Converting from an RPM-based Linux distribution to RHEL
As to the second part of your question - doing this without a subscription. The short answer is that you can't, as far as I know. And moreover, even if you could, once you are running RHEL you will need a subscription anyway to be able to download packages from Red Hat's repositories in order to install software via yum or dnf. Running RHEL without a subscription is kind of pointless, as you won't get software updates or access to new packages.
Red Hat do have some options for gaining access to a free subscription if you are using RHEL on up to 16 non-production systems, which you can read more about here:
No-cost Red Hat Enterprise Linux Individual Developer Subscription: FAQs
Now, up until a couple of weeks ago, I would have said if you wanted all the benefits of RHEL in a production environment but you don't want to pay for the privilege, you can look at other Linux distributions which are downstream re-builds of RHEL, like AlmaLinux, Rocky Linux, Oracle Linux or VzLinux. But the status of each of these as a like-for-like 100% identical re-build of RHEL is a bit unclear at the moment due to some source code access changes that Red Hat have recently implemented.
So whilst there's nothing stopping you going for one of these options, you should be aware that you might end up in the future finding that your system is not completely and exactly identical to the corresponding RHEL release, depending on what strategies and decisions the maintainers of these distributions make in order to keep producing them.
One final option you might have, if you are running CentOS 8 specifically, is to migrate to CentOS Stream. This is a distribution that is not intended to be an exact clone of RHEL, but rather is a target for future RHEL development. In other words, there are things in CentOS Stream that will one day go in to RHEL, but there are also things in it that might not, or at least won't for some time until they are known to be stable enough for inclusion. There are various tutorials on the Web for doing this, but no formal documentation from Red Hat or the CentOS project themselves.
I've also just noticed that you are asking about going from CentOS 7 to RHEL 8 - I think first you'd have to go from CentOS 7 to RHEL 7, and then do an in-place upgrade from RHEL 7 to RHEL 8. I don't think there's a direct path from 7 straight to 8.
Anyway, hope this helps ! If you have any further questions please let us know.
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You would need on prem repos created using dev licence for instance or cloud resource.
After that, you actually do not need a subscription to upgrade, as you are hosting your own repos.
I hate to sound obvious but try it.... pop a VM and upgrade it.
Advice - don't upgrade 
Tools method is often based on everything being from core repos to work e.g if you added some 3rd party repos, stuff might end up broken.
Deploy new and migrate the services and data in question.
Make your process repeatable and future proof, not dependant on 3rd party scripts/tools if you can
Otherwise pay up and let RHEL folks help you.
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This is great advice, and is absolutely what I'd do in this situation myself. I generally avoid in-place upgrades unless there is absolutely no choice for some reason. If you can build a new server on the OS you want, you'll have a lot less pain just moving your content, services, applications etc. across from the old server to the new server and then just updating your DNS to use the new box. Of course if you only have one server and cannot get another then you don't have much choice, so it all depends on your circumstances really.