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Swapping the parts out of the lower into another one?  Or just putting a different lower on the existing upper?

The answer to either is no problem.  Tearing down  built lower can be a PITA but I have done it twice and I did not have any issues.  The front take down pin is a PITA.  You need a slightly hook fine point object to hold the detent down and push the pin out.  I used a curved set of tweezers to do this.  Try a local hobby store to see if they have them.

The bolt release can also be tricky.  A narrow handled punch was what I used.  I set it on the roll pin and used a small hammer to tap it out.

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Just putting a different lower on the existing upper.

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No issue at all, go right ahead.

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Swapping lowers just involves 2 pins. Tool-less and painless 5 second swap.  

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Wrong

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[/div]It was always explained to me that the calibers must match between the upper and the markings on the lower; that's why most lower manufactuers mark the AR lower as Cal: Multi to avoid issues

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Nope. Manufacturers aren't even required to mark the caliber on the firearm - just mfg name, city, state and serial.
The caliber and model, if such exist, are recorded on transfer paperwork, but are not required to be marked on the firearm.

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Are you that positive? Please 'splain this:


PART 478—COMMERCE IN FIREARMS AND AMMUNITION
Subpart F—Conduct of Business

§ 478.92  How must licensed manufacturers and licensed importers identify firearms, armor piercing ammunition, and large capacity ammunition feeding devices?

(a)(1) Firearms. You, as a licensed manufacturer or licensed importer of firearms, must legibly identify each firearm manufactured or imported as follows:

(i) By engraving, casting, stamping (impressing), or otherwise conspicuously placing or causing to be engraved, cast, stamped (impressed) or placed on the frame or receiver thereof an individual serial number. The serial number must be placed in a manner not susceptible of being readily obliterated, altered, or removed, and must not duplicate any serial number placed by you on any other firearm. For firearms manufactured or imported on and after January 30, 2002, the engraving, casting, or stamping (impressing) of the serial number must be to a minimum depth of .003 inch and in a print size no smaller than 1/16 inch; and

(ii) By engraving, casting, stamping (impressing), or otherwise conspicuously placing or causing to be engraved, cast, stamped (impressed) or placed on the frame, receiver, or barrel thereof certain additional information. This information must be placed in a manner not susceptible of being readily obliterated, altered, or removed. For firearms manufactured or imported on and after January 30, 2002, the engraving, casting, or stamping (impressing) of this information must be to a minimum depth of .003 inch. The additional information includes:

(A) The model, if such designation has been made;

(B) The caliber or gauge;

(C) Your name (or recognized abbreviation) and also, when applicable, the name of the foreign manufacturer;

(D) In the case of a domestically made firearm, the city and State (or recognized abbreviation thereof) where you as the manufacturer maintain your place of business; and

(E) In the case of an imported firearm, the name of the country in which it was manufactured and the city and State (or recognized abbreviation thereof) where you as the importer maintain your place of business. For additional requirements relating to imported firearms, see Customs regulations at 19 CFR part 134.



   

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Looks like it can be on the barrel and still work. So you are safe with it not on the receiver. Also if you look at almost any revolver the caliber is usually never on the frame, its on the barrel 90% of the time. Same goes for bolt action rifles. Not cost effective to engrave each receiver for the caliber if they use multiple calibers for each receiver.