The cost of plastic foundation almost makes cleaning not worth it. At $1.30 you don't have to spend much time on cleaning or its not worth it.
I am not sure how to define "old" for plastic foundation. We have some that would be over 12 years old and I am not thinking of replacing it.
Very old brood frames have very little wax in them and makes it not worth trying to get the little bit of wax out unless you can press it.
If I do replace a plastic brood frame, it is pop out the old one, boil up the frames, clean the grooves and pop back the new waxed plastic.
Back to the topic, if I had to do it again, we bought 200 full plastic frames early in out venture into BK'ing, pain in the uncapper and also bees tend to wax them to the bottom of other frames, makes supers hard to split. Most of them are now in brood boxes.
Buy good quality smokers, price doesn't count if you light your smoker and 6 hours later is still going, only add fuel, no relighting.