Actually, I think it's slightly more complicated
than that: I have seen a BANK scene take place on
several different backgrounds, so I think when a
BANK scene of a particular season is made (most
of time they change every season of a show), the
cels must be kept in a drawer somewhere and they
are re-used when necessary - change of background
but same cels.
For example: Saint Seiya, when Shun uses his
nebular chain, the BANK scene in which the chain
makes a kind of spiral. I have seen the same BANK
scene on different backgrounds. I think in the
80's they could not kind of incrust the cels on a
background and had to take all the pictures
again - not sure though. What do you think ??