Just call them and have them send out a card that says "multi" in caliber field.
Factory upper and barrel are nice and light and well balanced. It is a very light factory barrel and since you do not have a grip below the action you want the end LIGHT so that you can hold it one handed and not change grip while loading mags with other hand (after putting in a left hand release which allows you to use thumb of left hand to depress lever while changing mags).
If you do change upper, and want a scope, I would suggest side changing to side charging upper since a scope on an SCR is going to be as low as possible and back further meaning charge hand will be cramped on normal upper.
although the gun uses AR accessories, besides balance being a bigger consideration due to grip position being further back and weaker, the other consideration is that that rifle shouldered will be higher and therefore all optics will need to be way lower than AR. If you use battery dependent red dot and flip up BUS you may want those BUS to lay super low so as not to block red dot, yet flip up above red dot ie not co-witness through, but completely above
I never even thought of the barrel weight without the pistol grip. Good thing to keep in mind when I go upper shopping. With the brass deflector I had to mount on the rail (lefty), no additional room for a BUS which also makes me cranky about the factory upper.
Have you tried the featureless DS-15? I don’t think this was around when we purchased our SCRs? Looks like it supports a fully conventional BCG vs the SCR and might also be easier to raise. I’m tempted to buy a 300 Blackout upper for the SCR and buy a DS-15 as my 556 gun.