The tense usage of "in the past", "during the past", "within these few days"

Hello, everyone:

For the phrase "in the past", "during the past ten years..." , and "within these few days" I've check my dictionaries and searched in the Internet, and I found that they tend to use "present perfect". So I'm wondering if they could also use simple past tense. Like:

In the past he has been a bricklayer, and now he is a teacher. (Can I say "In the past he was a bricklayer, and now he is a teacher."? )

There have been many similar cases in the past. (Can I say "There were many similar cases in the past."? )

So many extrodinary changes have occurred within these days. (Can I say "So many extrodinary changes occurred within these days."? )

During the past 2 months he has lost 27 pounds. (Can I say "During the past 2 months he lost 27 pounds."? )

Please help me with this. Thank you for taking your time reading my question.