want to make a new diskette same as previous except different rc.custom.gz

Forgive this stupid question.      I have a tomsrtbt diskette that I have 
built with various kernel and config changes.     I use this diskette on my 
laptop.   Now I want to make a new one that is exactly the same EXCEPT for a 
few changes in rc.custom.       I don't want to update the existing one, I 
want a new one.      I know I can do the whole buildit process with a new 
1/rc.custom.gz, but that will create a whole new install image from what's 
currently in my unpacked dir, and I am not 100% certain that I haven't 
changed anything.   (and don't want to change it back if I have).     My 
question is, is there some way I can clone the new diskette directly from 
the old one, e.g. dd?   and then update the rc.custom.gz.     If so, would I 
do it under regular linux or under the existing tomsrtbt?     Are there any 
gotchas?

John