If you don't want to change the order through SQL, then you can use the Sales History Load function (under Processes, Sales History) to change certain aspects of invoiced/posted orders (including the order date).
Note that this function was designed to allow new customers to 'load' existing sales history into Macola for historical/reporting purposes - it was not designed as a general maintenance tool for changing existing orders.
The OEHDRHST and OELINHST tables are pretty much 'standalone' so changing something in these tables will really only have an impact on sales history reports. I wouldn't want to change something that would make the order have a different $ value than the corresponding invoice or credit in AR and I also wouldn't want to change anything that would make an invoice reprint different then the original invoice.