Firstly, you'll either have to buy out everyone who purchased the items for a ridiculous amount or go to my second point. If you don't do this, you'll go the way of the server I used to dev and you will lose massive swathes of your dedicated playerbase. Yes, you artificially add more BM into the game but degradation is meant to pull money from the economy.
Secondly, mainly directed at @Food stamps, degradation is necessary in an RSPS because inflation will annihilate this game beyond repair in the long term. I used to dev (mainly UI and language interfaces) for the largest RSPS and you know what killed us? Besides Jagex finally getting the domain pulled, but the lack of cash sinks in the game. Our first idea was to make shops actually cost gold, then it was to make PvP armors degrade (idiot owner started RWT trillions so that didnt help) and then Barrows became degradable but oh no! - we still had a garbage inflated economy so then the most radical, and honestly our only shot, had to be employed - we reduced the cash stack of EVERY player by 10. You have max stack? Now you have 214.7m. That killed the game permanently and lead to Ikov and Alora or whatever it's called. Also, if @Jasonwere to implement PvP armors dropping, the market would very quickly become inundated with PvP armors and your investment would still be shit out of luck.
My suggestion would be to round up everyone like @Food stampswith PvP armors, give them either untradeable or degradable (leaning towards untrades) PvP items that you would always keep. PvP (maybe Barrows?) should have repair costs associated with them but with PvP items having both higher initial and higher ramping so that a 1% Vesta item would cost more to repair than a 1% Barrows.
If you don't want to make them untradeable, then @Jasonyou have to really consider flooding the market for a temporary time. Do you have any statistics of how much BM there is currently within player banks? More information is required to answer this question