I don't think its sacriledge.
Just as a note, there are two types of ar's.
The smaller AR's which are chambered for 9mm/45/223/556/50beowulf/458socom/50BMG/etc etc, and then THen the larger 308 ar's which are in 308 size calibers, and you cannot interchange uppers from one to the other.
I would suggest the smaller ar, because that is where the most options are - be they 223, 7.62x39, 9mm, 45, 50BMG, .458 socom, etc.
In the smaller ar's, any lower can take nearly any upper, although you may need different magazines, or a block that goes in your magazine well so it can take pistol magazines. For example, i could able to buy an olympic arms 10mm upper and throw it on my rock river arms lower. Except for colt. Because they don't like us.
Just know that lowers are (comparatively) cheap, around 1/3 the cost of the rifle, or less. So i predict that if you have more than one upper, you will soon have a lower for each one.
