Potty retraining possible?

Hi there...a few years ago we adopted two female chins. They came with their Marshall cage (I'll attach a pic). We've replaced all the plastic shelves with untreated kiln-dried pine, and made them a wooden hide as well (rather than the ferret tubes that were in there, Ewwwww). Our problem is that the previous owner would put the paper bedding in the corners of the shelves for them. Which of course taught them to pee there. So... They still do even though the bedding is different (Aspen) and the shelves are not only replaced with wood, but also in different places. It's downright gross and we don't know what to do. I keep it as clean as possible, and have for 4 years now,but there's times when it involves using a spatula to scrape the junk off the wood. There's never been mold, everyone is healthy, so I guess that sufficient... But... The urine has run down the leg of the cage for so long it's finally eaten through the metal,making it unsteady. Yes, a new cage is on the horizon.

So... Ugh. In the meantime... Any thoughts on how to retrain them? I'm afraid even a new cage won't change convince them to go to the bottom level.

I hope that pic shows up... The cage in question is the one on the right.

Thanks!

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