I would think that would be a thing. Were talking about jacking up Heavy into an Assault with Clan tech to the Endosteel bones. I'd would figure there new builds. Traditionally, only "C" and not "IIC" are refits of the originals with maybe the exception of the Orion C the Clan Wolf Empire is building.
The IICs get done in 2 flavors . . . one is a brand new design paying some sort of homage to the name of the previous design (the Phoenix Hawk IIC really stretches this, Griffin & Shadow Hawk are more the norm) and then you have ones that are rebuilds of the actual design (Orion IIC, Commando IIC and Highlander IIC to mind).
The C series comes in two flavors as well, the 3025 designs with standard everything else and retrofit with Clan weapons and nothing else (Atlas C, Marauder C, Warhammer C, etc) which is why most of them ended up lighter than the norm. The Orion C is a bit of a hybrid to the approach (we got a RS for it), most of the chassis uses IS parts that were already part of the design on the production line and Clan weapons for the full tonnage as well as punch. The second is designs like the Imp, Marauder II, Firefly and a few others, these designs are the Clan versions of what the Dragoons brought to the IS so they are the 'originals' that the dumb'd down versions we originally saw came from. I think the Gallowglas WD is in the same boat, but it gets a different moniker.
I am with Weirdo about what happened to most of the Warhammer 7As & 6Rbs in the early Clan's hands. They were one of the prime line machines they had on hand . . . some might have survived into modern times but IMO that would be extremely rare (you know, that proviso about how singletons might still exist into the next era) and I would not want to pilot them b/c they are likely patched together wrecks. Were Warhammer 7As turned into Warhammer IIC? Why? The engines are going to be different sized, the weapons are all Clan spec, the structure is going to be heavier, and many other changes outside of MAYBE the cockpit- and I doubt even that is the case.
So you backed that Warhammer 7A into a factory refit cradle on Strana Mechty to Clannerize your ride . . . its dismantled piece by piece and all the upgraded parts go back into that cradle to build your new Warhammer IIC. At the end of the process, you have that shiny new mech where you had a Warhammer 7A . . . but hey look, the Clan has enough parts to build a Warhammer 7A too!