I've dehydrated fresh, frozen, and canned -- even leftovers like rice and pasta. No problem with rehydration. In fact, dehydrated cooked rice is the same as "instant"; ditto for dehydrated pasta. Fresh veggies need blanching and you must be VERY precise with the timing or you will cook the veggies altogether before drying them. It's no biggie if they do get a bit overcooked, but then you're duplicating the effect of drying canned food (albeit without the salt, etc. of commercially canned foods). In fact, blanching fresh veggies is recommended before freezng them to halt the enzyme process and preserve color and flavor. Again, be careful about the amount of time they are submerged in boiling water to keep them crispy-fresh, not soggy. Rehydration of commercially frozen veggies is a snap and my family likes the flavors.
All in all, I would recommend eating the "about to expire" commercially canned goods in a stew, casserole, or other such dish. For future, if you need to decrease the space used for storage, then by all means go ahead and dehydrate commercially canned veggies and fruits, but frozen is MUCH better to dehydrate.
Also, there is the thought that canned fruits and veggies are good to store because you won't need clean water or an energy source to bring them to edible condition. Dehydrated veggies are usually not very easy to eat in their dried state. Fruits are incredibly good as high-energy snacks in their dried states, however.
Just my two cents' worth...