Like everything else, easily tapered but less easily widened. You can widen a pant leg by half an inch at most. Tapering is usually done along the inseam. If you have a vintage pair of pants talk to your tailor about taking them in around the hip along the outseam. Also, unless you want skinny skinny pants that border on leggings, don't taper below the knee. Take in as much as you need through the hip and thigh, but keep the circumference from the knee down. You'll appreciate not having mom jeans.
Easy peasy. Need HRD say more? Denim has a slightly distress look at the seams, which you may deem worth keeping (your tailor can do this; it's magic) but every other fabric will look the same no matter what fabric is used at the hem.