Thanks for your tips.
" If only LimeSurvey version and design template : no reason of upload directory for SURVEY updated
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I think then the overarching question is: How would limesurvey database on "Version 2.64.1+170310" respond to be put into an installation of version 3.26
Before: Installation + Uploadfolder coherent to Version 2.64
After: Installation on 3.26 + Upload folder like before, but the folder for templates is altered.
The thing is: If I went down route, I would also have to update another database with tokens, which gives tokens to users. Because that database holds like 4000 Tokens, but in the Limesurvey Database I see 14000 Tokens that are completed='N'. I would expect both numbers to be the same. And therefore, I don't really trust that other database.
So maybe instead of retrofitting two databases, I will just do the standard approach and completely import the two old databases, like I already practiced.