6L80, assuming ls engine.
all downshifts? how much load? at what revs? all revs? used for towing?
how is it on power upshifts?
many questions.
have you had it from new?
depending on make/model of vehicle it is in it may have an adaptive learning function enabled.
i would do some research and then ask the dealer to reset the adaptive learning function. the box will relearn to shift with current wear and tear, and your driving habits/style.
smell is the number one thing for me. looks like it was brown/gold when it went in. unless it smells burnt and is really dark, its usually ok.
the fluid level checking procedure is not that difficult. some dealerships suck ass/get their apprentices to do the service work. i would double check the fluid level before replacing the transmission/converter. it shouldn't be hard to find the procedure online.
in all honesty it sounds like a minor issue, and may not be worth the cost of replacement. depending on how annoying it is i guess.