There is always a small chance that a user who happens to be updating the category may get an error if you do it during day-time.
So try it in off-hours.
You said "descriptive" but I assume you mean the "key" flexfield for categories.
If you add 2 segments, it is important that you update existing categories and make sure they are valid with the new structure.
Also, if those old categories were used in Purchasing, you may get errors.
If you are trying to add segments to a category DFF, then you can certain do it during day time.
And it is quite low risk.
If the new segments are not "required", then you don't even have to go and update the old categories.
What navigation path are you using to make this change?
Yep - it is descriptive flexfield.
In that case, the change is a low risk.
As long as the new segments are not mandatory, you can do it without any issues.
If they are mandatory, just make sure that you enter the data for existing categories as well.