It appears that 375 UM brass can be made from 300 UM brass. If so, can it be made by fire-forming?

hmmm, yes, you can... form, then fireform, then trim .. might have a doughnut in the neck - can be done

Can the 300 UM brass be fireformed to 375 UM directly or does it need to be necked up prior to fireforming?

the 300 rum won't fit in the chamber -- you could size without expander and fireform without pre-trimming -- or, i guess, you could expand, tirm, fireform, trim

375 RUM loaded ammunition is available.
Not cheap but considering time and components needed to reform and fire form it may be a reasonable solution..

M

All that work and the expense of fireforming ?? Wouldn't it be just as easy to buy a few boxes of factory loaded ammo and then you have properly headstamped brass ??

Looks like factory ammo is the way to got in view of the scarcity of brass.

It's easy to make .375 RUM brass from either 7mm or .300 RUM brass. I do it with every one of the smaller cases that I find.
I made a long taper expanding plug and simply lube the smaller case mouth and expand it. Then FL size the case and reload it.
I've done it to over 100 cases and never had a failure.

I agree with the previous post. when 375 RUM brass was hard to get I got some Nosler 300 RUM brass and replaced the sizer button in the 375 die with a sequence of two tapered expanders, first from .300 to .338 then from .338 to .375 then put in the original button. It is noted above that the 300 brass won't fit in the 375 chamber. I never noticed a change in the brass other than the increase in neck size, but the final sizing would have formed the brass to 375 dimensions so they fit fine.
Another option that I tried was forming 375 RUM from 404 Jeffery brass. This plan didn't work well, unless you consider that I gave up on converting the brass and bought a 404 Jeffery.