move a primary key to a child

I want to add a child table to my table and using one of the primary
key of the table.
For example:

MyTable is

PKey1
PKey2
PKey3

I would like to move PKey3 out and add a child table as following and
remove PKey3 from MyTable:

ChildTable as folliwng
PKey1
PKey2
PKey3

MyTable will be

PKey1
PKey2

I just wonder what will impact in the future like reports, forms,
queries, data entries,
It seems that I need make a lot of changes for the application.
Any suggestions that should I continue to do this change?

Your information is great appreciated,