For an upcoming survey I need to change the design of the checkboxes to numbers, because we aim to transcribe the items from a former pencil and paper questionaire to an online survey.
I don't see a problem. Or a need to adapt an online survey 100% based on a paper and pencil survey.
I think people think too much of these things. And even if so, you are changing the methodology, there are at least some adaptations to be expected.
I would not bother with substituting the radio buttons with graphics of numbers, I don't think this will even impact on results.
What I would do is create the right scale for an array type question.
E.g.
Trifft stark zu<br/>1
2
3
4
Trifft keinesfalls zu<br/>5
So then you have the numbers over each column, but also the text. Should work just fine.