It's safe to do. In fact if you look at my tent mod I edited all the original buildings and developed a new building style to cause a unique look for some people. In fact if you click on save and use the same name it doesn't overwrite the original building it creates a copy in the documents folder under tiles.

Even moreso saving it with the original name will just replace that instance for your game. That means that if you make a change to K_Outpost_01 and save it as K_Outpost_01, then you will see your modified version in the game not the old version. If you delete your modified version you go back to the original version of the tile, no need to mess with the core files what-so-ever. (I recommend deleting the version when outside the game to avoid confusion with core-tiles if you practice this method).

Ok, I don't guess it, I did making change original and using all K_ect.... for kingdom faction after I done, I do save on same name as I loaded and I did checked override...indeed I had all those file in my  documents folder under tiles but then I start new game and play, I see nothing changed of building?

Sorry about that... all that does is duplicate the art

Okay here is a fix for you without editing core files.

Say you wish to change the merchant to something else. We do this

<ImprovementType InternalName="Merchant">

<ArtDef>Art_Merchant_New</ArtDef>

</ImprovementType>

then you need to change the art def to get the tile to work out like so

<ImprovementTypeArtDef InternalName="Art_Merchant_New"</ImprovementTypeArtDef>

<ImprovementTypeArtSubPack InternalName="Art_Merchant_Kingdom">

<SupportedBuildingStyle>KingdomStyle</SupportedBuildingStyle>
<TileDesign>SomethingDifferentThanTheOtherName</TileDesign>
<ConstructionTileDesign>K_Construction1x1_01</ConstructionTileDesign>
<DrawnIcon>Improvement_Merchant.png</DrawnIcon>
<DrawnIconConstruction>Improvement_Merchant_C.png</DrawnIconConstruction>
<Thumbnail>K_Merchant_02_Thumb.png</Thumbnail>
<Medallions InternalName=""> 

<All>K_Merchant_02_Thumb.png</All>

 </Medallions>

</ImprovementTypeArtSubPack>

<ImprovementTypeArtSubPack InternalName="Art_Merchant_Empire">

<SupportedBuildingStyle>EmpireStyle</SupportedBuildingStyle>
<TileDesign>F_Merchant_01</TileDesign>
<ConstructionTileDesign>F_Construction1x1_01</ConstructionTileDesign>
<DrawnIcon>Improvement_Merchant.png</DrawnIcon>
<DrawnIconConstruction>Improvement_Merchant_C.png</DrawnIconConstruction>
<Thumbnail>F_Merchant_Thumb_01.png</Thumbnail>
<Medallions InternalName="">

<All>F_Merchant_Thumb_01.png</All>

</Medallions>

</ImprovementTypeArtSubPack>

</ImprovementTypeArtDef>

This will change it to the tile design with internalname "SomethingDifferentThantheOtherName" 

so for your tile you edited you need to go into it's xml and change the internal name to something different otherwise you will end up with a duplicate art design for the tile.

Again, sorry for the mis-confusion the test I did earlier was adding something to a tile design, not really changing the design itself.