Can we change the account number, once given and entries been posted, so that it could be put under another heading?
Yes - but it can take a significant time, if there are many postings to that account, as Navision will update all the entries, where this account was used (not only GL entries, Posting groups and other places will also be updated accordingly).
In CoA, change the Acc No, system will ask for confirmation first. Do it in off-peak time, as during the update the system may become less responsive to other users.
updated: Oops - Erik managed first while I was slowly typing... I understood the purpose is to change the accno in such a way it "moves" to another group of accounts...