graphic isomorphism – Are the lambda expressions isomorphic (without type) semantically equivalent?

"Isomorphic" is defined as having the same form of syntax trees and the same variable links. However, the names of the variables can be completely different. In other words, it is to say that we have a graphical isomorphism between the two expressions (the graph is the syntax tree with the variable link included as edges).
The intuition behind this is that assuming we are given the isomorphism, it is always possible to change the name of the variables and get the other expression.