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 ??