If the Dyes are the same, or insignificant difference you might keep the new ones for your Dual. (Giant 4 > Ancient 4 not worth upgrade)
Dye type
Description
Lv. 1
Lv. 2
Lv. 3
Lv. 4
Lv. 5
Giant Dye
Increases one basic stat of your character and decreases the other one.
+3 -2
+4 -3
+5 -4
+5 -2
+5
Ancient Dye
Increases one basic stat of your character and decreases the other one, increases resistance to one attribute.
+3 -2,
+5 from attr.
+4 -3,
+10 from attr.
+5 -4,
+15 from attr.
+5 -2,
+20 from attr.
+5
+25 from attr.
Legendary Dye
Increases one basic stat of your character and decreases or increases the other one, increases resistance to one attribute.
+3 -1,
+5 from attr.
+4 -2,
+10 from attr.
+5 -3,
+15 from attr.
+5 -1,
+20 from attr.
+5 +1,
+25 from attr.
Dye Merchant can "delete a symbol", if it was applied with x10 Dyes, you will lose half of them... if it was Exalted x1 Dye symbol you lose it.