I won't name the source, he'll probably speak up himself. Is this a reality? Is it true that if I change an upper on my AR15 I need a mfg license? Or if I swap barrels on a Contender, Encore, Mauser, or Blaser that I am jail bound for not having a mfg license? Is this really being enforced?

I don't have the money to challenge the ATF&E in court. I don't want to get thrown in the pokey for swapping uppers.

That source is wrong.

How about if I rebarrel a mauser rifle with a new 8mm barrel?

The lower receiver is the only serialized part on an AR type system.
If you can mail order the part without an FFL then you can also swap it as you please.

I was able to buy everything I needed to assemble my own AR back in 96 the only part I had to get at a dealer was the striped lower receiver

So as long as I am doing modifications as a private individual and not selling a service or product, I am probably ok.

I feel better now.

Whoever told you that was desperate for your business. You, as an individual, can build as many guns, change, rebuild, customize as many as your heart contends and your pocketbook can afford.

If you engage in buying, selling, building, etc as a regular course of trade, then the alphabet boys and girls will probably want a cut of the till.

As an 07 manufacturer if I do any work not directed by the customer, it is considered "manufacturing". For you, no such thing.