I want to update an A cdt with another B cdt which is subset of A cdt. Is there

You can create a expression rule and pass cdtA and cdtB into it. Use type!cdtA() and map the cdtA fields with cdtB fields.
Ex:
type!cdtA(
id: if(APP_isnull(ri!cdtB.id),ri!cdtA.id,ri!cdtB.id)
name: if(APP_isnull(ri!cdtB.name),ri!cdtA.name,ri!cdtB.name)
gender: if(APP_isnull(ri!cdtB.gender),ri!cdtA.gender,ri!cdtB.gender)
age: ri!cdtA.age /* If age field is present only in cdtA and not in cdtB */
)

This way you can update the cdtA efficiently preserving the non-common fields also.

Lets me know if the information was helpful.

You can create a expression rule and pass cdtA and cdtB into it. Use type!cdtA() and map the cdtA fields with cdtB fields.
Ex:
type!cdtA(
id: if(APP_isnull(ri!cdtB.id),ri!cdtA.id,ri!cdtB.id)
name: if(APP_isnull(ri!cdtB.name),ri!cdtA.name,ri!cdtB.name)
gender: if(APP_isnull(ri!cdtB.gender),ri!cdtA.gender,ri!cdtB.gender)
age: ri!cdtA.age /* If age field is present only in cdtA and not in cdtB */
)

This way you can update the cdtA efficiently preserving the non-common fields also.

Lets me know if the information was helpful.