We had this discussion at ATT, and I did drag up Calgar and his 2 models available for sale on
GW
https://www.games-workshop.com/en-US/Space-Marine-Marneus-Calgar-and-Honour-Guard
https://www.games-workshop.com/en-US/Space-Marine-Marneus-Calgar-Lord-Macragge
If he has 2 datasheets use the one thats most appropriate, obviously.
For models that came supplied with smaller bases which later received larger bases (Marines, Stealth Suits, Crisis Suits, etc) there's been no direction from
GW Im aware of that stipulates those models must be ripped off their old bases and put on newly-purchased ones.
the ATT discussion was about the Tau Commander, for which there are 2 datasheets: Tau Commander and Tau Commander in XV86 Coldstar Battlesuit. Nothing in the indices stipulated what model the commander uses, but codices and literature in the past have made it clear that Commanders use different models of crisis suit interchangeably (Enforcer Suit, which you make with the Tau Commander kit or the previous Finecast kit; Or the newer XV8 Suit; Or even the older XV8 suit). Before 2013, your only option for modeling a Commander was the standard XV8 model. The Coldstar wasn't available until 2015.
Thing is, Calgar's datasheet specifically states he's a TERMINATOR in Terminator Armor. So it would not make any sense for the Calgar in Power Armor model (on a 25mm base) to be used as the 40mm Termie model.