Foxtrott: Hello js!
The colored part of the rear cover can easely be changed (just a few screws). The part is identical on all 21" MX (from MX3000 to 4200) and should be available at your dealer.
Hello JS
I'm afraid to say that Foxtrott is incorrect.... the covers all look the same but have a different screw alignment on the inside.
The black tube housing at the extreme rear of the set is a seperate part (unattached to the coloured portion), and needs to be screwed to the coloured section before attachment to the television.
The problem arises when you try to attach an older set's rear housing (such as the MX3000's) to a newer coloured section - the screw holes on the rear section will not fit the screw holes on the coloured section.... so be careful when purchasing a new rear!!!
The only possible way around the problem would be to aquire a new tube covering as well - an MX4000/4002 cover will fit your MX3000, but you'll need to remember to swap the green sticker holding the serial number and other important information!
Hope this helps.... take care when taking yours apart though! One slip and you could be in for a shock!