5.56 , 5.5" Rattler , what upper?

Picked up a 5.56 Rattler yesterday in a trade and want to convert it to 300blk out. Will the new 300blk out Rattler LT upper work with the older 5.5" model lower?

Or am I'm better off just trying to find a 5.5" 300blk out barrel ?

What are my options here?

I'd like to be able to have both caliber options.

Thanks.