1) you can perform it anytime, up to you. before or after Year end close or month end close.
But note down that after this process, from next moment BusinessUnit dimension will be mandatory for all data entry (which will be new)
It will be much better if you do it at month end or year end and inform users that from next day dimension will be mandatory.
2) this change will not reflect back to SubLedger level i.e. to modules. it will effect your GL balances only.
for new entries which will be done after modifying COA structure (i.e adding BU dimension as mandatory) you will get dimensions everywhere
if you want to see changes of historical data i.e. addition of BU with each entry, that means run SysDatabseTransDelete and delete everything and redo all entries.