No. All v1.1.0 saves are compatible. However, if you start a new game after installing the DLC, you’ll see new Abyss characters appear in Chapter 2.
A new "Everyone" option allows you to change the appearance of all characters simultaneously. Fire Emblem Three Houses -NSP--Update 1.2.0- -2...
010055D009F78000 (Base) Update Title ID: 010055D009F78800 Fire Emblem Three Houses -NSP--Update 1.2.0- -2...
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For those with the Expansion Pass, Update 1.2.0 adds the side story: Fire Emblem Three Houses -NSP--Update 1.2.0- -2...
You can now deliver lost items to Rhea and invite her to tea parties after completing the quest "Esteemed Company" starting in Chapter 5.
A new option allows changing the appearance of all characters at once.