Hi Kern,
Length increment doesn't impact much.
at dev system, please change A length to 8 and activate it.
later reactivate object B s well.
As you said A is not used in DSO/cube then data deletion is not required.
Lets try to check whether it have any inconsistency thru RSRV.
Once every thing looks perfect then you can move changes to Qua/prod after getting approval from business.
Thanks.