changing the valves in a terror bass is fairly easy, the one thing to look out for is the screws that hold the top cage section on are very cheap and prone to the heads getting chewed up. I swapped mine out for some black cap head bolts which are much more robust, I can't remember exactly what size but could check when i get home.
there is a thread on here somewhere that goes through the whole process of swapping the valves , if i get a chance to find it i'll add a link. but once the top cover is off it's take the valve covers off (bayonet fitting) and then pull the valves out carefully, then make sure the new valve is properly aligned and it should just push in, covers on and off you go.
I put some lower gain valves in mine but never really gelled with it, at some point i'll put it up for sale.
Matt