if they're just metal, just put them grip end down in boiling water, remove grips, replace grips. if you're gonna cut them, get a pipe cutter, they're like 7 bucks if you look. if they're carbon or something then cutting won't be easy, if possible.
For the xwing pole with the leather wrap, its not too hard. Run the handle over a hot air gun for a minute or 2 then get a freind to help you pull the grips off. From here you will want to cut the leather. Mark how far down the bmx grip will go and then cut round this mark. Just to say, a standard bmx grip will not tighten onto the pole. I have tried this before. What I would recomend is either try to find a grip which goes straight on or get some scott ski pole grips. They do lots of differnt sizes and there really good. If you do get a lockon grip to fit you will need a bar plug to fit in the end. A mtb bar plug will fit if ou shave it down.