u means micro. Similar to k = kilo, m = milli. So 1000u means 1000uF, Farads being the unit capacitance is measured in. 1000x10^-6 Farads is how big those capacitors are. The 10^-6 is due to the u.
If they are bypass capacitors you should be able to replace them as long as the value isn't too far off. If the caps do something else other than bypassing it could be a problem. THey're probably for bypassing if they are that big.
Do you know what "recapping" is? If you don't (or if you do but don't know anything about it), Google it. Apparently a lot of people go through speakers and stuff replacing capacitors.