Right, and I am comfortable with that. I am an Aqualung dealer, and have no qualms about swapping the necessary parts to convert a Core 1st stage to a Core Supreme 1st stage, even thought they have not bundled them as a "conversion kit." But it will still be marked EN250A >10°C. If searching for the right phrasing, maybe that is better described as an authorized modification.
More important is not to start moving around 1st/2nd pairing that were not tested together. While if each was part of a CW configuration in the first place that will probably be okay, it is not definitive unless that new pairing was also tested at some point. And having a CW 1st stage does NOT automatically mean that ANY 2nd stage used will "inherit" that property from the 1st. It is both components and systems that provide freeze resistance.
I can only speak to my knowledge of the Atomic product line (especially since that's the parts I happen to have in front of me). Atomic offers the Z2/Z3 and B2 first stage with and without a coldwater kit. It's a factory option to have it installed, not a different product (Core vs Core Supreme). The coldwater kit can be installed during rebuild (or at any other time I suppose) and the >10C markings are on the close out ring that is replaced as part of the coldwater kit, not marked permanently on the regulator.
At this point, we're just debating semantics, but it's not quite the same thing as swapping parts from another product line as the reg with the coldwater kit is exactly what Atomic tests. Functionally, I doubt it's any different than swapping out the components in the Core regulator.
I certainly agree that, at bare minimum, you'd want to ensure that all components have been tested to that standard. Pairing an EN250a rated first stage with a non rated 2nd stage would not pass on the performance to the 2nd stage.
I struggle a bit with buying in that the EN250a rating is as a system, however, as other than bundled kits from a distributor, EN250a rated regulators are generally sold as a matched first and second, but not with a matched octopus. If it was necessary to have the octopus be included to maintain the EN250a rating, I would think it'd have to be sold as a complete package.
I'd love to read the standard itself, more out of curiosity as an engineer than anything, but I'm not about to fork over the $250 it costs to buy access to it. As such, everything I'm saying is only my inference from reading secondary sources, so I could be totally wrong.
Interestingly, Apeks has a discussion on their website where they talk about having their octopuses rated separately for use to EN250a standards. Also, to an early mentioned question about what EN250 rates the use of an octopus to, they reference a minimum operating depth of 100 feet. Their octopus ratings are with the intent of exceeding that.
Now I'm extremely curious to look at the actual EN250 and EN250:2014 standards side by side.