no there not, if you have standard sky with just one stb then in all likelyhood a single lnb would have been fitted.
as for changing an lnb, tbh yes its reletively easy task to carry out. but having said that in todays climate, e.g stringent h/s rules and regs, blame culture, etc i'd be very reluctent to offer advice that may mean working on ladders/at heights and would say leave it to the pro's
edit- oh,and by the time you've factored in the cost of an lnb,decent cable and connections... plus ideally after replacing the lnb you should be checking dish alignment which will require a sat meter though you can by cheapish meters by the time you've done all that its not gonna cost you a great deal more to get a pro in