Hi Antonio,
Unified Expression Language is - as the name implies - a unification of both expression languages existing before JSP2.1/JSF1.2.
So the ${expr}-Systax is still valid as is the #{expr}-Syntax and there is not need to change all ${expr}-expressions to #{expr}-expressions.
But both usages differ in meaning and therefore they are not allowed everywhere.
The ${expr} syntax means that this expression should be evaluated immediately. So as soon as this expression is reached it gets evaluated.
The #{expr} syntax refers to deferred expressions. These kind of expressions are evaluated at certain points in the JSF lifecycle.
You can find more information here:
http://java.sun.com/developer/technicalArticles/J2EE/jstl/
and:
http://java.sun.com/products/jsp/reference/techart/unifiedEL.html
According to what you've posted I'd assume that the ${expr} syntax should be just fine for your purpose.
Wolfram