I got crazy Brave & upgraded one of my 10.14.6. accounts with this update, but I did it booted from a 10.14.6 Clone tp Apply it to the Non Booted 10.4.6 volume.
Seemed to go quickly, was slow on startup from the upgraded volume, lots of BeachBalls, but I had dozens of things open at restart, I noticed in Activity Monitor that the Launch Services & Spotlight indexes were both being rebuilt & using lots of CPU%, took about 20 to 30 minutes for things to settle down & normal speed returned.
One note is somehow Safari got damaged & would not Open, luckily I could get a Dev download of just that.
This was on an iMac with 32 GB of RAM, so I surmise if you only have 8GB of RAM tied to a 5400 RPM drive that it could be excruciatingly slow for hours.