Just a little extra information for you Gabor!
Sketch Automate includes a couple of specific functions I think should at least help!
The first is
Change Local Symbols to Library Symbol
What this will do is allow you to swap out all of the local Symbols within your document with those that live in a Library.
If for example, the Symbols in the Library reference Text Styles within the same Library then these should all get updated accordingly as well.
The second is Find and Replace Text Style
This will let you select a Text Style and replace it with another (to make it easier to tell the difference between the Library and Local Text Styles, I would recommend adding say OLD to the end of the names of Text Styles in the local document just to remove any potential confusion there.
There are various other functions which may be of help as well but I’d recommend having a look through the Automator menu under Plugins as there are quite a few.
I hope that this helps!