Here is my complete MS Flow/Power Automate example showing how to send alert email when a specific column value is changed. In the example, the current value is compared with the previous value by retrieving item's versions from the history using [Send an HTTP request..]
Please see the screenshots and explanation below
MS Flow triggers by [When an item is created or modified], and
at the beginning of the Flow, a check is made to verify if the item was modified and then proceeds

Detail 1

Expressions 1 and 2
1. varPrevComments
if(empty(body('Parse_JSON')?['d']?['results'][1]['Comments']), '', body('Parse_JSON')?['d']?['results'][1]['Comments'])
2. varCurrentComments
if(empty(body('Parse_JSON')?['d']?['results'][0]['Comments']), '', body('Parse_JSON')?['d']?['results'][0]['Comments'])
Detail 2

Please let me know if you have any questions.