How to build a simple data comparison rule?
OK, I would then recommend Business Rules to build that kind of data comparison rule.

1- Create a first Variable "Value from Entity field" to store the original value of your field.

2- Create a second Variable "Value from Entity field" to store the current value of your field (i.e. the saved value after updating node)

3- Create an Action "Send an email"

4- Create a Condition "Variable data comparison" to compare the two variables you built in 1 and 2. Available variables are listed, use {{your_variable_name}}. Use the right operator, here I needed != (different from).

5- Inside your Condition, add the Action "Send an email" (in section "Items to execute if condition succeed")

6- Create a Rule based on an event "After updating Entity", define the context (entity type and bundle, here content and content type) and insert the Condition you built in 4. You should see a nice flowchart with your Rule label (white), then your Condition label (blue) and if YES, your Action label (green).

That's a start.

Hope this helps.