.Dat Swaps

Can other players see the .dat files you have changed while in game? or is it only viewable by the person who made the change? Like if i change how shiva looks will others players see normal shiva when i cast it or will the see my swapped shiva? any info would be great.

Its only visible on the PC you change it on.

It's only visible on the PC you change it on, and it changes the .dat for every single iteration of it on your screen. So it wouldn't just change your own Shiva, it would change everyone's Shiva. The only time that isn't the case is when you use something like Wardrobe, which only changes your character (though it's still only visible to you).

See, how the game works is on the server-side, it has all of the various positions for all of the .dats on your screen at any given time, as well as all of the monster AI and pretty much everything that happens in the game. All the .dats do is read those positions and show what they do on your screen. You can, for example, change all Spiders in the game to look like Dragoon Wyverns, but it won't change their damage or defenses or even their attacks. Worst-case-scenario it might read a file that isn't in a specific .dat you've changed, and the character might react in an odd way (think Wacky Waving Inflatable Arm Flailing Tube-Man).

so with the update today do i need to remove those .dat files i put in to update the game or can i just update it without replacing the new files with the ones i removed?

You just update it - it may flag some of the mods you have put in and overwrite them during the file check, but not a lot of the armor usually gets overwritten. The UI mods will always flag tho, especially since they are adding the mog sack this time through. Anything the game flags it will replace with the original dat, but it should be simple enough to put them back in unless you have a bazillion that are you've modded that get replaced and have to go through one at a time - in which case it is still simple, just tedious ^^