In regard to a rebore of the barrel, it probably isn't a big problem in the short term for a semi-auto, casual-use rifle (e.g. a plinking gun) but for any rifle that was firing auto or rapid semi-auto or something you needed to rely on to put meat on the table, it's going to have a negative effect on accuracy and precision.
Keep in mind I'm not a gunsmith or armourer so this is from my own understanding of things (I know some of the guys here are gunsmiths/armourers so please step in and correct any dumb mistakes I make!)
Apologies in advance for telling everyone how to suck eggs but a little exposition is going to be helpful
Two simplified definitions for the non-gun people: -
Accuracy - the ability to hit the target where you want to hit it.
Precision - the ability to consistently hit the target with accuracy.
Because the barrel works as a heatsink, rapid firing can build up enough heat to cause the metal to soften and in extreme cases the rifling can be worn out much faster than normal and reduce the rifles accuracy over time. As I said, not a big problem if it's not firing full-auto or firing hundreds of rounds in rapid semi-auto in a short space of time.
The primary safety issue at this point would be that obviously, a thinner barrel will heat up faster than a thicker barrel and in the long term, this could wear the barrel to the point where weak spots develop. Should something like a blocked/clogged barrel or double-loading occur during firing, it might cause the barrel to split/burst at these weak spots with the potential to send metal fragments into the face and arms of the shooter.
The second issue with the barrel heating up is that it can affect the barrel harmonics. Harmonics describes the vibration of the barrel during firing and is important for longarms as the effects are related to the square of the barrel length.
Way back, people used to call this "barrel whip" but that's misleading because the greatest vibrations don't occur up and down like the motion of a horse whip (although there are smaller vibrations that do), they move in a circular pattern. Obviously, this has a very strong negative effect on precision and a thinner barrel is not going to be as rigid and therefore as resistant to barrel vibration as a thicker barrel.
Manufacturers will typically find a specific barrel thickness that makes the best compromise between weight and cost without negatively affecting accuracy and precision.
You often see precision target competition rifles with heavy or "bull" barrels specifically because they are trying to minimize the affect of vibration on precision but you don't necessarily want a heavy barrel on a hunting or military rifle.
By reboring a 5.45mm barrel to 5.56mm, you will probably screw up the harmonics severely because 1. the barrel thickness was optimized for 5.45mm ammo and 2. the barrel is a little less useful as a heatsink which in turn, interferes with the harmonics. As far as I can find, a typical 5.45mm AK barrel has a diameter of about 0.57 inches (approx 1.447mm) while a typical 5.56mm AK barrel has a diameter of about 0.59 inches (approx 1.498mm) so you're already starting with a barrel thickness that is not optimized for 5.56mm and by boring that extra 0.08mm out of it, you're reducing it's diameter from 0.57 inches (1.447mm) to 0.53 inches (1.367mm) taking it even further from it's optimum thickness-to-ammunition calibre ratio.