If a period is closed, a transaction shouldn't be changed at all, but if a transaction is merely reconciled there ARE certain changes that should still be possible:
2. Category and/or splits
3. Memo descriptions
4. Merged (but only into another non-reconciled transaction!)
5. Cheqbook date (but not bank date)
6. Ref (check no).
I would not allow changes to the amount, to delete it, or to merge it with another reconciled transaction.