Yes & no. Length to shoulder for the .375 Flanged Magnum is the same as that of the .375 Belted Rimless Magnum - 2.4". What makes it look different is that the Flanged has a longer neck than the rimless, .440" vs .350", which results in the Flanged having a longer overall length of 2.940" vs 2.850" for the Belted Rimless. Ditto the Super .30 Flanged and Belted Rimless. Twenty-five years ago the Brits were high and behind converting Flanged Magnum rifles to Belted Rimless because ammo and brass for the Flanged could not be had at any price. Mac is quite right that going in the other direction would seem to require chamber sleeving though.

Rechambering a double rifle can sometimes make a lot of sense, especially when the original chambering was especially dumb. Always plan on re-regulating, although it sometimes isn't required. It isn't particularly expensive anyway. A double in something like .458 WM could easily be rechambered and re-regulated to one of the Flanged .450s, such as .450 No. 2 NE, for $1,000 or so I imagine.
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