Can a ar15 lower that was originally received as factory pistol lower that was then converted to a
Rifle, be put back in to pistol form if the pistol buffer tube is reinstalled or is it stuck as a rifle? I know it’s a dumb question.

You need a competent legal answer if you are betting your life on it and want to avoid possible jail time.
As i understand it, once a lower has been assembled as a rifle, in the eyes of the ATF it's ALWAYS a rifle no matter what you hang on it.
That;s why its recomended that you buy a pistol diesgnated lower and keep it in that configuration.
OR buy a regular lower and HAVE IT ENGRAVED as a Pistol Lower even if it's the very same lower reciever.

I AM NOT A LAWYER and ignore legal advice from me or anyone else here who is NOT A LAWYER or certified legal firearms expert. ATF SHOULD have guidance on this online.

Why not just take a rifle lower, tear it apart and use it for a spistol, after getting it engraved as a pistol lower?

Because as I understand the law, it's ILLEGAL.

Why risk it, new lowers are cheap enough these days?

Yeah that’s what I was thinking. If you call the manufacturer of the lower they say it was sold as a pistol lower. Only reason I bothered to ask, Looks like I will just order bcm pistol lower just to be on the safe side. The whole is just to hard to get a correct answer.

Once it's a pistol, it can always become a pistol again.

If it starts life as a rifle, it is forever.

But with the prices of lowers, I think it's easiest to just get another lower.

Other than my AAC PDW lower, I couldn't remember which other one started off as a pistol so I just built up another pistol lower. They're so cheap right now.

A stripped lower can be built into either a pistol or a rifle, which is why you need to be 21 and able to legally purchase a pistol in order to buy one. My point being that all stripped lowers can legally be built as a pistol. No marking or engraving is required. If you build it as a rifle, you can always convert it back to a pistol.