Aw man, that was so easy! Just turn off the main water supply, let the pipes empty, cut the pex pipe with the $15 tool, slam the Sharkbite connectors on the swapped pipes, and turn the water back on. Bam! It took 5 minutes, and 4 of that was just getting up the nerve to turn off the water.
Now all I need is for this ancient master shut-off valve to stop dripping at a rate of 4 drops per second.
Uh.
Huh.
What was that about sleeping dogs?