You should be able to use older scores built on older templates without any trouble, as long as you use instances of Kontakt Player that have soundsets that match those template versions. For example, if you are opening a score built on VDL Template 5.2.5, then you will also need to have the VDL Soundset 5.2.5.xml file in the correct location on your hardrive (per the VDL readme) and you will need at least one instance of Kontakt Player in your playback configuration where VDL Soundset 5.2.5 is assigned.
If you have significant works in progress the recommended approach would be to re-input non-pitched percussion instruments into the latest VDL Template (currently 7.0a). This will give you up-to-date mapping that has been significantly improved upon since templates were designed for Sibelius 5. It will also give you access to VDL instruments in the latest version of VDL, v2.5.5, with everything ready to go.
My rule of thumb for converting to a new template versus sticking with an old one depends on the type of music I am working on and what I plan to do with it in the future. For example, I have many cadences I've notated over the years. These are built on Templates 5.2, 5.2.5, 6.0, 6.0a, and 6.0b. I typically leave these alone because usually I just look them over, maybe edit the number of bass drums I have according to that year's line, and make a PDF.
However, if I am working on a show arrangement, I will make the effort to migrate to a new template. I started next season's marching band opener on the 7.0 template, and when 7.0a came out I made the effort to migrate to 7.0a. This was a worthwhile investment of my time because the custom mapping and other tweaks I make to this opener score will also be used at the templates for the rest of the show.