If by the 'ceiling rose' you mean the circular plastic plate that has some 'lego blocks' on it to make the various connections, you can do away with it. Pull the fuse/trip the breaker that protects your light. Make a note of which wires are connected to which in the rose. You will only have to remove 2 (the live and neutral to your existing fitting; these will be replaced by your new ones). If the new fitting is metal, you may have to connect an earth wire. This will go to the earth in the ceiling rose (either a bare coper wire or with a yellow/green sheath). Take the 'lego blocks' (they may be all brass) out of the plastic plate. Your new light fitting should come with some sort of mounting plate or bar. This should screw into the holes where the plastic ceiling rose fitted. Connect all the wires back into their respective 'lego blocks'. Wrap insulating tape around each lego block (remembering you may have 2 or 3 wires on each block). There should be enough room to fit all the wires etc either in the new fitting, or you may have to push some of it into the void in the ceiling. Fit the new fitting to the mounting (normally with 2 screws and usually very fiddly). Get someone to support the weight of the new fitting. Finally, put the fuse back in, switch on and if it doesn't go BANG and it lights up, you've done it right. If not, find a friendly electrician!!