bibtex - Change format of urldate

Thanks to Giacomo's feedback, I found another solution for this specific problem:

The problem is somehow more related to Citavi and Citavi is not as clear or transparent as LaTeX. For future reference of other people who might encounter a similar problem: Although Citavi asks to provide the date in the format urldate={dd.mm.yyyy} you can type it in as urldate = {yyyy-mm-dd}. The export does not make any problem. Strangly, by just changing one entry, all the other entries are exported accordingly. My fault not trying it out earlier. I should have tried it out earlier.

If you are using biblatex package you can use

\DeclareSourcemap{
    \maps{
        \map[overwrite]{
            \step[fieldsource=urldate,
            match=\regexp{([0-9]{2})\.([0-9]{2})\.([0-9]{4})},
            replace={$3-$2-$1},
            final]
        }
    }
}

in your preamble to reformat urldate field of input *.bib files.

for forcing the formatting in BibTeX, I use the double braces, e.g.:

Urldate  = {{15/01/2014}}