Can jean waist sizes be altered?

With denim pants, waist gaping can be an issue, but an experienced jeans alteration service can modify the waistband to nip it in a bit. Just make sure you don't take the jeans in any more than one to one and a half inches at the waist, as this will alter the pocket placement and front shaping. Not to mention that to keep the appropriate line, you'll probably need to adjust your hips as well. Wearing a belt or finding a pair of jeans that fits your hips properly is generally easier (and less expensive).

When jeans fitting services make changes to the waistline, you'll almost certainly need to make changes to the parts that conceal your backside (aka seat of the pants). If you have to take the waist in, for example, there will be excess fabric on the seat of the trousers that need to be removed (let the seat in), and vice versa.