My quick description how I wrestled with Shimano bottom bracket in order to replace bearings rather than spending money for new cups/bearings set.
The story began with BB-4600 which seems to be fine for 6 months, and then suddenly, at one evening after ride I realised that whole drivetrain is not spinning freely as it should. Quick inspection revealed that component which apparently should be quite robust died quietly. Left bearing in bottom bracked spins but with grinding feeling. After quick calculations was obvious that I need another one in order to keep my wheels rolling. Eventually decided to make a small experiment. I decided to replace bearings in old cups, and slightly upgrade them in the same time.
Step 1: Find decent bearing!
The first step was to find out what size of bearing I'll need. After quick measuring/googling I realised that Shimano makes life hard for those who wants replace just a bearings. The problem is simple. Almost impossible to buy (decent quality) bearing in size 25x37x6mm, but it's broadly available bearing 6805-2RS (sometimes known as 6805-RD) which is 25x37x7mm. I've decided to buy set of ceramic bearings from China trough ebay, as I wasn't sure about result of the whole operation. They cost me £20 including postage so not too bad price after all for (semi)ceramic bearings. I've clicked "Pay" in PayPal and the fun begun.