Say I had an item I had imbued, is there a way that I could reforge it to keep those same properties but have 100% fire/poison damage?
I'm not a reforging expert but I think the reforge has to be done before the imbuing. Also standard materials must be used to reforge.
Basically you craft an item, reforge it with a runic, higher runics give more properties lesser runics give less properties, if its under the 500 Imbue Weight and is 5 properties or under you can imbue it or edit the weight on the 5 current properties.
Here is a table on Reforging from Stratics: UO Stratics *New* | Runic Re-Forging Honestly, I think your best bet with reforging is using a low end runic to try and get a single property above max intensity. (IE: Shadow Runics will bump out -15 Mage Weaps.) There is no way to gurantee a property and I don't think you will get a 100% fire weapon from reforging. If you have Valorite Runics and such, I would suggest popping on TC and trying it on reforging before you waste 2-3 charges on a weapon. I still think those are better for just using to make weapons.