A regulator must be able to get the same pitch out of a string every time he plays it. So if you are planning on having your harp regulated, plan on changing strings at least 2 weeks in advance of the regulation, and then tune it many times and play the strings so they are absolutely stable when the regulation is done. Nylon strings take much more time to stabilize than gut strings, especially in the lower octaves. If you change nylon strings in the third, fourth, or fifth octaves, I would do that a month before the regulation.
Yep, my tech says change strings a minimum of two weeks before regulation as well.
Once the strings settle in they may stretch a little over time, but not that much. Mostly, as the strings age, problems with regulation come from the strings getting pits where the prongs on the disks press against them. If you can easily feel the indents on the strings from the disks it’s time to change strings.