Well...I never worry about guarantees/warranties on hardware...and I don't buy whole systems.
My question is...have you talked to anyone at Dell about what happened and what you'd like to do?
I have never interpreted changing the O/S as a "change", since I thought that such warnings only applied to people like me who like to open the system and fool around with components.
I suggest you talk to someone at Dell, first and foremost.
Let's understand something...there is only one C: partition on any system. No such thing as creating "another."
You can dual-boot operating systems, but only one of those will be on C:, while the others will have different drive letters.
As to your question...does it make sense to have an operating system installed that you don't want, don't use, and don't need? If not, why even consider dual-booting?
IMO, there are very few things that go wrong with a computer system...where correction requires "support" from a box-maker. Between forums and the various informational items available at Microsoft and other websites, it's all there waiting to be read, understood, and executed.
And when all else fails with a problem, a clean install manages to solve any O/S situations
My two cents