Well, you should upgrade, 6.2 is also pretty much EoL. I hope, you have at least the binary patches applied.
https://liferay.dev/blogs/-/blogs/creating-liferay-security-binary-patches
That said, you need to basically follow an article like this one. I searched a bit but couldn't find a similar article for 6.2
https://help.liferay.com/hc/en-us/articles/360018175011-Installing-Liferay-DXP-on-Tomcat-8
I would take a plain Tomcat 7 and compare it with the Liferay 6.2 bundle. Then I would apply the changes to the 8.5 instance. lib, lib/ext, some config files. And I would try that without the DB. I would test it with the HSQL database first, then use some other, new DB for testing.
Only when it looks good would I test it with a copy of your productive DB to minimize errors.

Thank you, Christoph!
 In fact, moving from Liferay 6.2 to 7 is already considered in my road map but now I got an urgent request to move out of tomcat 7 to a supported version.
Also, Liferay 7 CE  does support Oracle DB! which might cause a problem or might takes a longer time.

i totally I agree with you , migration from 6.2 to 7.1 is a project by itself and is not on our scope for now, but moving out of unsupported version is my responsibility. and that's the reason why I'm looking for a shortcut.