Yes, you'd need to use CB Auto Actions for that. You'd act on the after frontend profile update trigger and compare var1 to var3 of a field in your conditional. Example condition as follows.

[var1_username] Not Equal To [var3_username]

var1 is the new user object and var3 is the previous user object so this allows you to compare the 2 raw values of a profile update field.

Where can I find variables of the dropdown list trigger ?

You can find trigger variables in the below tutorial.

www.joomlapolis.com/support/tutorials/120-api-usage/18358-using-cb-triggers

It's missing some of the newer triggers in CB, but the majority are documented there.

Ok this consideration is important and helpful but I think that is not correct to change the status of this topic in SOLVED.

When we can have moderator approval for fields that are different from image fileds I presume the topic il solved ??

We've nothing further to add to this topic. It's already going off topic as is. It was marked solved to close it so everyone can move on from it. When the feature is implemented we will announce it. Marking SOLVED is more friendly than marking CLOSED, but at any rate I've changed to it CLOSED. We do not revisit month old topics to update them so your scenario in question would never happen (unless it's tagged with a ticket number).