I guess this answer has been asked before but I cannot filter out a solution to my particular case so I try it with a fresh question .

I am having a finished survey from last year and would like to start a new one next month. This new survey is identical to the old one, but some questions are easier (and more reliable) to answer if I provide the answers from the last survey. Is there anyone who can point me in the right direction how to solve this?

I have LimeSurvey 2.6.2, PHP and Javascript development are no problem if required. The survey is token based with a unique token for each participant and all surveys.

You can provide data via token attributes. These attributes can be accessed during the survey.
www.limesurvey.org/manual/Survey_participants#Manage_attributes
So you need to extract the answers of every respondent and create attributes. Which then can be filled with the data again. You than can e.g. put the information in the question text (Last time you answered this question that way... etc).

Thanks, sounds good. Can I create token attributes from the API? I have over a thousand participants... Couldn't find a att_attribute method so far...

I don't know about the api, but you can upload your participants via CSV. This should be quite quick, even with over a thousand participants.

Problem is, I already have the participants added to the survey. Can I just reimport them with the additional attribute?

Add Attributes via LS. Export tokenlist. Add attributes data to csv.
Import tokenlist. The problem is, that you might need to delete tokenlist and than import tokenlist again.
That can be a problem, when the survey is activated and running. But in your case should be no problem.
I would test everything with a copy of your survey. Your LS version can contain bugs related to the tokensystem.