I have a Kyoshiro's Soul that dropped when the character was 49, but the sweeping wind abilities are a big part of my build, so now I'm paragon 211 and everything I have is great except the belts stats.
If I reforge it, will it reroll into the SAME belt? Or into a random legendary belt?
Is there any danger of me losing the abilitys, or is it simply going to give me new higher level stats now?
If you reforge an item it's as if it dropped again - it'll be the same item and the same level, but with newly re-rolled stats. That also means it's a 10% chance to be ancient and 90% non-ancient.
That being said, re-rolling an item is among the most expensive recipes there is, and Kyoshiro's Belt is a really common item. The stats are low because it's a level 49 belt (and it'll stay level 49 if you re-roll it). Re-roll is more advised for weapons and/or items that are really rare see this list for which item on which slot is considered to be rare). You should just use the belt for now and gamble for it at Kadala (or potentially re-roll rare belts in Cube, though for those common items on normal armor slots which cost 25 blood shards at Kadala I recommend gambling over cubing). You can extract the power into your cube - that's the best use for low-level legendaries, as the item level doesn't matter for Cube extraction. But I'd advise against re-rolling.