Cannot change Creditor Invoice date

Problem

Cannot amend a posting date of a creditor invoice in a different financial year. 

Current setting for finance is:
GL year is 2018
Reporting year is 2017.

Solution

Since you are working in 2018 the System will not allow you to change the date to a different financial year. 
The best way and easiest way is to print the transactions before abandoning it and create it with the correct date again.
Alternatively, we could ask the Customisation team to run some scripts to copy the transactions for you but it may take time for them to do and will be a chargeable task.