If ever you need to open the estimate again for editing/amending, change the estimate back to Draft in the status drop-down. You would also need to set the Estimate back to Draft if you needed to do additional take-offs for the Job.
NOTE: be careful of unlocking the estimate and changing dollar values, if our contract value ceases to match your signed contract value all invoices will be incorrect values.
For more info on the overall process of going from estimate to job see the link below