Im not sure it's a done deal that Lloyds are going to come back to St William House. The assertion in the Argus that "...the Cardiff office is being refurbished..." is not a direct quote from Lloyds:
https://www.southwalesargus.co.uk/news/24087889.lloyds-bank-close-newport-offices-move-cardiff/Lloyds Bank told the Argus that this move will cause no job losses, and the Cardiff office is being refurbished to provide "a state of the art, modern workspace".
It may turn out to be the case, but I suspect that they are going to play the SWH landlord off against the market for new space in central Cardiff .
The landlord will be highly motivated to retain Lloyds because it's highly unlikely they will find someone else who needs that amount of space and it wont be easy to let that building to multiple tenants. Lloyds will want a relatively prestigious building as it will be their only HO location in Wales. They will also demand the highest environmental credentials, and although the truth is that re-use is likely to be better than new build because of the embedded carbon, it probably doesn't work like that when corporations report the environmental impact of their leased estate.
I'm not convinced that SWH can tick these two boxes no matter how much it is improved. So, If I had to guess I would say that it will end up being redeveloped into a residential or mixed-use high rise scheme.