Swapping different AR caliber uppers

[Hypercussion]

I have a 2009 Olympic AR that is chambered in 5.45 x 39. It's fired tens of thousands of rounds reliably with regular cleaning and maintenance as needed. I am starting to have some issues and really need to overhaul the upper. The lower is in perfect shape. I recently ordered a DPMS upper in 5.56 for another project and it made me wonder...why couldn't I convert my 5.45 AR into a 5.56 AR? The Olympic is a Mil-Spec AR-15 platform as is the new DPMS. My pmag 5.56 x 45 mags from my other ARs fit the mag well perfectly and if replace the upper with a complete 5.56 upper (BCG/CH etc..), it should in be a 5.56 carbine. Right? I am hoping it's that easy as while the 5.45 got me through some tough times by being able to shoot cheap surplus ammo (always non-corrosive), I do prefer 5.56/.223 and I'd be just as happy ordering another upper (no FFL required!) for my project and eliminating one type of ammo to buy and stock completely (only 5.45 gun). And I could probably get decent buck for the old Oly 5.45 upper on Gunbroker.

Appreciate hearing if anyone has experience doing something like this or info that might be helpful. Thanks.

[Clint]

Yes, that is part of the beauty of the AR platform.

Provided there is nothing special about the lower, you should be able to drop that complete 5.56 upper with BGC and mags right onto the lower and rock on.

[RearwardAssist]

some of the lowers that use for shooting com bloc surplus use stronger hammer springs for the harder cups only thing i could think of that may be an issue.

[Hypercussion]

Thanks guys for both your replies. I figured it would be a go but always good to check. I have good working knowledge of the AR platform (ex spec ops) and do all my own maintenance and most repairs except anything where it's obvious a pro gunsmith is needed. But I always ask when unsure. I have heard of stronger hammer springs on 5.45 and 7.62 x 39 guns to accommodate Soviet ammo, and also guys having to replace firing pins prematurely on American made rifles that shoot surplus ammo. Looks like for what I intend to do though, there aren't any major concerns and I'm going into this expecting that I might have to do some tweaking. Thanks for the help!

[eodinert]

What issue are you having that warrants a replacement upper?

[Hypercussion]

I'm having a continual problem with extraction. About a month ago I had a fail to eject at the range. Spent round was stuck in the chamber pretty tight, had to run a wooden dowel down and tap it out. I replaced the extractor spring and O-Ring and it ran fine for about another 250 rounds and now same problem again. So while I am t-shooting this issue, I have a new complete 5.56 upper sitting here for another build (still waiting on some lower parts for that project) and got to thinking, why couldn't I put the 5.56 upper on the AR I'm having issues with and shoot it while I'm diagnosing and repairing my 5.45 and waiting for parts for my new build. I probably won't get rid of the 5.45 upper, but it's the only 5.45 gun I own and if this all works out, it's one less type of ammo I have to buy and stock.