You can change a symbol after it has been used in at least one schematic. The tricky part is that you need to close gschem and restart it for the new definition to be used.
Try not to move pins in a symbol. This causes problems in schematics already using the symbol as the nets do not connect correctly anymore. Changing attributes of pins, on the other hand, does not impact schematic capture. You can also add or change attributes of the symbol after the fact.