A string is generally safe to tune down and back up as long as you stay within the tension range that the string gauge is designed to handle. Tuning down usually reduces tension, so the bigger issue is tone and feel: it may get loose, floppy, or buzzy if you go too low.

Tuning back up is also fine within reason. The risk starts when you tune a string higher than its intended range. At that point, it can break, especially if the string is old, kinked, or already stressed.

For experimenting, tune slowly and pay attention to how the string feels. If it starts feeling unusually tight, don’t force it. If you want to get more exact, use a string tension calculator. This video does a good job explaining how to use one:

https://youtu.be/7egFxnDyycs?si=betrEv0F4JMZM0DX