If you wish to keep your harp sounding resonant and bright, I would recommend renewing the bass wire strings every 3 years or so. Particularly with gut strings, you will gradually break and replace your strings so they are naturally renewed over time.
If you decide you would like a complete change to a heavier or lighter string gauge on a Norris harp, this is possible with perhaps just a few minor adjustments to the regulation of the semitone levers.
There are now several optional string types available including Heavy, Medium and Light Gut and a new choice of Heavy, Medium or Light CONCEDO Gut which are premium quality coloured gut strings from Bow Brand providing distinct white, black and red colours which are particularly helpful for professional players performing in different lighting conditions. Carbon ( or Fluorocarbon ) strings are also an option which provide good stability and longevity ( fewer string breakages ) ... and a particularly bright tone.
If you are considering a complete re-string, it can be best to have this carried out at the workshop if possible, to ensure all those expensive strings are consistently fitted and the semitone levers are checked and regulated if required.