Yes.
By definition, you can't convert between proper and improper fractions. You can convert improper fractions to mixed fractions, and vice versa.
Dissimilar fractions may or may not be proper fractions.
Some similarities of proper fraction and improper fraction are that they are both fractions.
Yes, every unit fraction is proper fraction because a proper fraction is a fraction in which the numerator is smaller than denominator. So the set of all unit fractions are also proper fractions.
40/48 is a proper fraction. Proper fractions can't be improper fractions; it wouldn't be proper.
7/6 is an improper fraction. Improper fractions can't be proper fractions. It wouldn't be proper.
4/6 is a proper fraction. Proper fractions can't become improper fractions.
No.
Mixed fractions can't become proper fractions. Mixed fractions are greater than one.
4/5 is a proper fraction. Proper fractions can't become improper fractions; it wouldn't be proper.
Improper fractions can't become proper fractions.
Improper fractions cannot become proper fractions.