If a newer Lightroom Classic version tells you that a Catalog needs to be upgraded (because if was made and worked on under an older software version) - this upgrade is done and then saved to disk as a new copy with a different name. By default, this name will show the LrC version number concerned, at the end.
The Catalog that is being upgraded FROM, still remains behind on disk as well. So if that older software is still present in the computer, it can be told to open that again just as before.
The source images and folders that this older Catalog references are the exact same ones that the upgraded Catalog as used by the newer software is also referencing. They are physically shared across both, in other words, and everything starts out looking the same in relation to those within both Catalogs: library organisation, edits etc.
Further work in either Catalog will now cause them to diverge. A Catalog that is compatible with a newer version cannot be back-converted so that an older version can open it, this works in one direction only.
If you want to go forward with the latest version (as that develops over time) under a subscription plan, then Lightroom 6 becomes redundant. At some point it may come that your hardware / operating system will no longer support it, or that some cybersecurity consideration makes using this no-longer-supported software inappropriate.
If the older software is being discontinued, its respective Catalog can be removed without damage to the newer Catalog. Of course, you would need to leave all the imported images and their folders just as they are.
Otherwise you should consider any usage of these two versions as 100% independent except that you would need to be careful not to delete any images from disk when removing from one Catalog, or reorganise anything on disk, since the other Catalog may be silently relying on those same images remaining where they were. YOu would get no warning : a bad situation IMO. So I advise: accept the subscription - which is objectively good monetary value in my judgement - cease using Lr 6 altogether, and don't look back. You can then access the latest features constantly, and a product that is under active development. So any functional issues - and these do arise - are going to receive a new corrective update, which will never happen with Lr6.
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