Hi Rizu,
If you want to track also disposed fixed assets, you have to acquire them, depreciate as in your old system and then post the disposal/scrap transaction, which can be realized through an ordinary fixed asset journal. That is, even if you sold the fixed assets, you do not necessarily need to create a free text invoice for that purpose but can rather use the fixed asset jorunal.
Then the big question is how to setup the ledger accounts for the dispoal transactions and here it depends on what your customer wants. A good starting point is what has been setup in the contoso demo companies but note that this is typically only a starting point because many customers want to differentiate between Profits/losses for the different transaction types, which basically reverse and clear the prior fixed asset postings. So, you will need to analyze with the customer how they want to have their postings tracked and do some testing.
Best regards,
Ludwig