I cannibalize old pieces all the time. I cut out parts and sew them onto current unit. Sometimes this can come out awesome. Other times it comes out good enough to get by untill your next unit.

Use sewing thread color that matches hair color. If you want a better hold then put some kind of fabric glue on the thread (on inside of cap of course) and the sew will hold nicely

I do this all the time. You will gget good at doing this. When cannibalizing pay attention to the hair direction of the part you are taking from the old unit as to where you willbe sewing and what is the hair direction where you will be sewing

Sometimes it will come out not how you wanted it to . Just cut threads try again with another cannibal part from other unit. I just get another part and try again.
This has come in handy many times. I am almost as good as a surgeon. Do not try on front hairline. This almost never works. Sides are easy , back is easy , any little bald spot etc easy stuff