Dumb (?) USB Flash drive question

I'll admit I didn't phrase things very well, but there are two distinct
actions going on here.

1. Since Microsoft has seen fit to not yet offer to upgrade my system to
1903, for no reason that I can discern, I am seriously thinking about
upgrading it myself, thus the downloading of the current 1903 version.

2. I just acquired a large USB drive, and want to make it bootable, for
reasons not really connected with #1. Thus the thought to use the Microsoft
manual download process to make the USB drive bootable, rather than just
download the ISO.

My understanding is that if I run startup from the USB drive WITHOUT
booting from it, in other words from a running 1809 Windows system, that I
will be able to perform an upgrade, and not a clean install, which would
happen if I booted from the USB first before running startup from it.