I maintain mainframe systems and very often I would like to restructure code to improve it, but the requirement is to make the most minimal change that achieves

I maintain mainframe systems and very often I would like to restructure code to improve it, but the requirement is to make the most minimal change that achieves the fix or change required. Mostly because the code is 30 years of patches and minimal changes which is not well documented and is extremely fragile and not well understood.