Can a Rem 700 Sendero II 300 RUM be converted to a 300 Win Mag ?

At a minimum it would need a new barrel.

But you can purchase ammo loaded to 300wm levels I think, or load your own at 300wm levels.

Same bolt face, different follower/mag box/spring.  The receiver rails may be a bit wider in the RUM, I can't remember.

cheapest solution, sell it, buy the one you want.

I've heard of .300win-mags being re-chambered to .300RUM, but the inverse cannot be using the same barrel.
Remington Factory Loads the .300RUM in three power levels: I is 30'06 power, II is .300WM, and III is the full house Ultra Mag.
You can easily duplicate those loads if you handload....then again, maybe you have damn good reason to go .300WM.
Any gunsmith should be able to re-barrel it even with a $100. take off barrel without any difficulty.

Reload down to 300 WIN MAG ballistics....