The flames on my stovetop are showing increasing bouts of yellow flame (incomplete combustion of the propane, i think) - replacement time?

Sounds more like you need to clean the stove. Take the burners and parts out and clean them real good. Probably clogged with food from over flows from the pots. The air-to-fuel mixture ratio is impeded due to dirtied lines to the burners.

my 70 or more,year old stove needs like once a year to have the burners removed and soaked on hot soapy water,and brushed out ,reamed with pipe cleaners or some such thing,then run under hothothot tap water,,your gas pilot flame may need a bit of tweaking to restore the uniform blue flame like a bit more air to combust the unburned yellow stuff

yep but should nave done it when they started to change color !!!

That could help or just taking them off and cleaning them out would also be a huge help.  And check the gas line to make sure there is no clog point in it; like dirt, bent or collapsed line both could also cause this problem as well as the shut off is partly shut off!  Mother's cat did that on her's so I had to wirer it so the cat could not turn the flow down again once I found out what the cause was to start with

Not replacement time:  cleaning time. Take your burners apart and muck them out. Use a thin wire brush to get into holes and thoroughly clean them. That's not incomplete propane combustion--it's probably food residue or corrosion. 

No but burners need cleaning.  Not a big issue

Debris clogging up some of those little holes or, if your stove has them. The little burner caps can move slightly and cause that issue.

Intermittent yellow is normal as dust and debris enters the flame.

You should never need to replace burners unless you physically damaged them.

Likely all you need to do is clean the debris off the burner.

the metering jets [orifices] are replaceable.  does require your propane dealer to do this [unless you have a gas plumbing license, this is NOT a DIY project].  -- grampa

I would suggest "maintenance time"!

The burners and associated parts might need to be cleaned. For more info see this link:

https://homeguides.sfgate.com/causes-burner-flame-...