I purchase an additional safe recently.  It was a deal Costco was running and has an "EMP proof" digital combination.  I've looked up the docs and it indicates they were tested but we all  know the value of these claims.  I've considered getting a locksmith to change it out to a manual model but the digital type is very convenient.   In case the things go pop and the combination lock goes with it, does that leave me with cutting it open to get to my rifles?  I've seen images online of gun safes cut open (most after fire) but no information on the process.  Is it just a matter of putting a grinder and a bunch of wheels in my EMP trashcan and hope I never need it?  

There is very, VERY likely not enough circuitry in your safe to serve as an antennae to be damaged by EMP.  You are much more likely to have trouble finding grid power to operate your grinder than you are to have EMP related issues operating your safe.

+1 on no grid power,you can change the lock yourself to a regular dial or get one of the combos,they are kinda costly.

I changed my digital only out to a combo....took about an hour all told.

I think this is the route I'll eventually go.  I have a redundant solar setup stored and protected.  Would hate for the first thing I have to do is use most of the power to grind open a safe.

Unlikely EMP would cause a problem, but electronic circuit boards in general...
I have dual electronic, combo dials. If I only had one, I would have a mechanical lock.

One thing to consider is there are very very few nuclear attacks on the US in a given year but lots and lots of thefts.

I've had a digital Canon residential security container for 29-years now. When I lived in California I was in-and-out of that thing four or five times a day as I kept weapons, jewelry, and cash inside. I also have an AmSec burglar safe with a five digit analog dial lock and thick walls. The "little" safe weighs more than the gun cabinet.

It takes me less than two seconds, in the dark, without glasses, to open my residential security container.

It takes me about 90 seconds in a well lit room, with my glasses, to open my burglar safe.  

Following a nuclear attack on the US you'll have possibly 2 or 3 minutes to seek shelter as additional surface nuclear weapons are detonated as they follow the EMP weapon inbound. Within 10-15 minutes of the first nuclear weapon exploding on US soil our fleet of boomers will be coming close to the surface to respond with our own application of liquid sunshine on the attacking country. Silos will be opening in the midwest for the ballistic missiles who will finish the job with a couple hundred more enemy cities leveled. Within the first couple of hours there ought to be several hundred million dead and within a week close to a billion.

As you wonder how much radiation the fallout contains ...

It typically takes robbers about two and half to five minutes to empty a residential security container with hand tools. There's a dozen YouBoob videos to show you how but basically you put a 1/4" drill hole in the 10-gauge steel and take a cutting chisel to open the hole. With power tools I could cut a residential security container in half than you can read this post. A air chisel would slice through the steel with easy cutting a foot of metal every couple of seconds.

FPNI

(im conjecturing here quite a bit). EMP is LONG wavelength so it needs LONG antennas to collect it, like dozens of yards long. The power grid is basically a network of miles long wires that make a great antenna for this, they would concentrate the power and then blow out transformers, plugged in devices, etc by blowing fuses, transformers, etc. Your gunsafe is maybe 6 feet tall and 6 feet wide. (a very large safe), the metal shell isnt nearly large enough to get a decent EMP charge.

There was a mythbusters about EMP strikes back in the late 90's and basically if i reall correctly, a "direct" EMP strike of catastrophic size caused the car to die, then start righ back up. They illistrated the lack of absorption due to the length of the "antenna" (wireing harness/car body).

Look at your manual. My digital/bio-metric lock has a keyed lock behind it.

The logic board for my electronic lock shit the bed, it was inside the door so I had to hire a locksmith to break into my safe. I had him install a dial type combination lock to replace it. Never again…

That sucks.

Gun store guy tried to convince me to buy electronic, cause the dial can get jarred and the numbers be off.

I ignored his advice...

Can you pretty much change any safe electronic lock with a mechanical one?

No, my cannon can not be. But its pretty easy to pop the inside door panel and see if the "lock" lines up with the hole for the cable. If it does it can be changed. In my case I could add some metal in the right places and attach a dial lock. But its not just a matter of swapping it out.

I opened mine up to check it out, when I saw it was a NO. I did some measurements to know what and where to attack it to get it open. In the case of my safe it would be done with a cut off wheel, to cut off the tab on the electronic lock that stops the mech for turning. There maybe an easier way than the cut off wheel but it would work.

Wish I could help. My Winchester safe has a digital lock but comes with a key to manually open it.

Better safes (Residential Security Containers) like Liberty and Fort Knox and many others will be using standard foot print locks.  I replaced some with a Securam safelogic xtreme which functions both as a digital lock for convenience, but also as a mechanical lock in case the electronics die for any reason.  Set both the electronic and mechanical codes as well.  

Costs a lot more than just slapping a basic mechanical or electronic lock on it, but I can open it quickly, and have the mechanical lock as a safety feature, so it is worth it to me.

I got broke into and the idiots chopped the dial off my safe with a hatchet, they didn't get in, I made a new dial with a protractor out of a cd and went through all of the permutations of the combination but they bent the shaft so the dials did not run true, I finally used 2 crowbars to spread the dooor and used a spring hook to line up the discs to get it open.
dials are not fool proof.