Take Magnesium Citrate, 400mg for 1 month. A gentle try to move every day, do not push or strain. If not, take a gentle laxative daily like Milk of Magnesia or "Smooth Move" herbal tea. During this month try upping the Mg and see if that has any effect. I see no ill effects from daily laxatives, and their are really bad effects of doing nothing or straining.
If that doesn't do it, and it probably will, the next month take 1 Potassium Asporotate(TM) (400 mg, for chelate) by Solaray (daily as necessary, but not for more that 1 week). In my experience, after 6 years of permanently constipated' that will do it.
Pre-bioltics (fermented foods) are very helpful, they're already partially digested.
Give your body plenty of time to adjust to each change in diet.