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For your site, yes I think it would be difficult. Because you have so many modifications there is a very large risk you would lose them and not know how to fix it.

I think it would be better if you do not update. You should start from a completely new installation of kiwitrees 3.3.63.3.5. Create a new folder in your kiwitrees directory and copy all of the files for 3.3.63.3.5 into that. Then got to the URL for that directory (perhaps http://nobility.pro/genealogy/kiwitrees335) and run the setup. During setup use a new database, not the existing one.

Do NOT add any custom code. Import your family history as a GEDCOM file (export from old database). Then check all configuration options and make sure everything works correctly before looking at customisations.

When it is all running as you want it, you can start to copy the custom code.

Any custom modules will need some re-writing. I can talk about that when you do it.
Changes to the theme should be done as described here: https://kiwitrees.net/faqs/customisation/customising-themes/ Do NOT add any changes directly to original code files.

When at last you have done all of that, you can remove all the old files from http://nobility.pro/genealogy and move all the files from http://nobility.pro/genealogy/kiwitrees335 up to http://nobility.pro/genealogy

If you have only a few users, tell them to register again. If you have a lot of users you can copy the user tables from the old database to the new one.

Yes, it is a lot of work. But that is because you did not update at every new version as recommended. I cannot help that.

Nigel
My personal kiwitrees site is www.our-families.info