Not sure if what you're discussing is the same proposals I'm (vaugely) aware of, or if those have been superceeded again, but my understanding is slightly different.
Network Rail's Welsh Route Study (March 2016) gives a figure of 13m passengers using Cardiff Central (CDF) in 2013 and estimates that will rise to 23m by 2023 and 32m by 2043. It claims "A modernisation of the station is rapidly required" to increase capacity for both passengers and peak-hour trains. To acheive the latter, they suggest a new west-facing bay platform (platform 5, but to me the new bay would logically be platform 4 with the current platform 4 becoming platform 5) and extension of platform 0 into a full-length platform for class 800s. Expanded circulation space to cater for the increased passenger numbers is also, apparently, needed.
The Welsh Route Study includes an "Artists impression of the redevelopment of Cardiff Central" by powelldobson architects. I have seen further artists impressions from the same set (I think in a Wales Online article) and having looked at all of them I have come to one rather alarming conclusion:
they essentially plan to demolish almost the entire station. The frontage with the 'GREAT WESTERN RAILWAY' lettering would survive, but the booking hall behind it would be essentially GONE (I don't think they are planning to move the frontage forwards, it would just stay put). Even the clock on the top of the booking hall appears to be missing on the artists impressions. The wing that currently houses M&S (and has a car park on top next to platform 0) would also be demolished and replaced by a 'greenhouse', with a similar 'greenhouse' on the other side of the frontage. On the platforms, all current buildings and the canopies woul be demolished and a Reading-style transfer deck installed. ALL of that (apart perhaps for the M&S bit; don't know if that is a later addition) is part of the listed building as far as I'm aware. In my opinion, by developing these plans Network Rail and powelldobson architects are showing an unacceptable disregard for the listed buildings system. I have uploaded three of the powelldobson impressions to my Flickr alongside some of my photographs for comparison purposes:
I'd be very supprised if they are planning to move it, they just want to demolish most of it. If they were to move it brick-by-brick, I'd suggest moving the whole lot to the St. Fagans muesum; but it's far too big for that to be practical. I did read somewhere (I think it was a Wales Online article) that widening one or both subways was an option, as an alternative to the Reading-style transfer deck above the platforms.
How is the station actually constructed? Are the subways tunnels through a solid embankment? Or is the whole thing essentially on a bridge with empty space (or staff only areas) beneath? I tried asking Network Rail and they refused to answer. If there is empty space under there, then I would support widening the subway that accesses the lifts (not the stairs one, that's one of the wonderful features of the listed building) to increase passenger space and avoid all the demolition.