General

[quote=KiritoSAO]Do you get magnificent souls by chance from turning in 10 shards or do you have to get a higher level shard to get magnificent souls? Also, can you replace a soul on a weapon with another soul? If so, how?[/quote]

1. Magnificent souls are gotten by chance from turning in 10 shards. Note that some bosses don't have a Magnificent version of their boss soul (Such as Pianus, Ephinea, Prison Guard Ani, Mu Gong)
2. You can, simply scroll the weapon with a new boss soul. The new boss soul will replace the old boss soul. The old boss soul will be lost forever once you replace it.

[quote=AnthonysWill]It's probably a magnificent soul. Magnificent Boss Souls are the only tradable boss souls, along with every Murgoth Boss Soul.
Magnificent Souls has a [b]chance[/b] of giving you a better bonus stat than the regular untradable boss souls.
The bonus stat is randomly selected from the following: % Att, % M.att, % boss, %Ignore DEF, % Critical rate, % All stat, or +1/+2 all skill levels.
Unfortunately, It's random on what bonus stat you get from a Magnificent Boss Soul, a Regular Boss soul shows you what you get(Radiant/Potent/Flashy/Beefy/Etc.)
A Magnificent boss soul doesn't.
Plus, Magnificent Boss Soul skills are usually better than the regular boss soul skills.
Magnificent Boss Soul Magnus>>>>>>>>>>>>Regular Boss Soul Magnus by a large threshold.[/quote]

Do you get magnificent souls by chance from turning in 10 shards or do you have to get a higher level shard to get magnificent souls? Also, can you replace a soul on a weapon with another soul? If so, how?

Ok to best explain it. Basically the ONLY event soul is Murgoth's. All the rest you see in shops are the Magnificent or equivalent souls, those are tradable among others, and they give special summons exclusive to that type hence why they're expensive. The other ones are given randomly when you get 10 shards of a certain boss. As to what bosses give them there's too many to name, but to put it mildly there's a lot.

[quote=KiritoSAO]If they're not rare, why do they cost so much (eg hilla/arkarium/zakum)? o.o I'm confused.[/quote]

It's probably a magnificent soul. [b]Magnificent Boss Souls are the only tradable boss souls, along with every Murgoth Boss Soul.[/b]
Magnificent Souls has a [b]chance[/b] of giving you a better bonus stat than the regular untradable boss souls.
The bonus stat is randomly selected from the following list: % Att, % M.Att, % Boss Damage, %Ignore DEF, % Critical rate, % All stat, or +1/+2 all skill levels.
Unfortunately, It's random on what bonus stat you get from a Magnificent Boss Soul, a Regular Boss soul shows you what you get(Radiant/Potent/Flashy/Beefy/Etc.)
A Magnificent boss soul doesn't.
Plus, Magnificent Boss Soul skills are usually better than the regular boss soul skills.
Magnificent Boss Soul Magnus>>>>>>>>>>>>Regular Boss Soul Magnus by a large threshold.
Magnificent Empress/Magnus/Murgoth Souls are probably the most expensive boss souls since they provide the greatest increase in stats.
Murgoth is the only "Boss"(NPC) with tradable regular souls, so his souls are expensive as well. He can only be obtained from an event, and that event is over.

If they're not rare, why do they cost so much (eg hilla/arkarium/zakum)? o.o I'm confused. Also, do the normal/easy versions drop these soul shard things or is it just the hard versions?

Not rare, in my opinion. And they're not event items. You can get them at anytime. But i recommend farming them on 2x drop. I got so many von leon and zakum boss souls i might have to drop some.

They're not that rare. Killing a boss with some drop rate e.g. Greed Pendant/Big Spider will probably get you a few soul shards when they die.
After you collect ten soul shards, you can trade them in for a soul of that boss, although it comes with a random selection of stats.