Looking at installing a new 9” 300 BLK barrel on my Gen 1 MCX to SBR it because back then I didn’t know that you should always wait a couple years before buying a new Sig product. The rifle has had the BCG upgrade. My understanding is that I need, in addition to a new barrel, a new bolt head, appropriately sized Gen 1 handguard, and that should be it. Is this correct?

Yep you’re on the right track . The new barrels require a new bolt that’s called a tapered bolt on the box . With the barrel and bolt you should be good to go . I did what your doing only back wards and it was a pain I have a virtus and at the time the only barrel I could get was a gen1 barrel so I had to get the old style bolt which was a pain in the butt and I way over paid . I know the rails from gen 1 of virtus are different but it seems you already know this . The mcx makes a great 300bl sbr . I run mine with 6.75’’ barrel and Q thunder chicken silencer very cool and quit set up .

I believe the Legacy has a longer op-rod, so you’d need a Virtus length op-rod assembly as well.

I can check for you when I get home, but I’m using a 6.75” Virtus barrel in a legacy receiver so I can use Lancer CF handguards.  I threw the entire Virtus BCG/recoil assembly in as well.

The barrels are interchangeable without modification

Sorry, you’re correct, op-Rod lengths are identical.

Awesome, looks like I'm on track. Thanks!

Oprod length only matters with the 5.5” rattler barrels.

My go-to rifle is a Legacy with a Virtus barrel and transitional handguard.

Love it.