Are you wanting a record for each
Or could you just have an Estimate Type and switch it when necessary

Thank you Alex for your reply,
Each estimate is a very detailed event with multiple  levels, rooms and different assemblies with associated quantities etc. When an estimate is completed it will result in a bid document, which may or may not result in an approved contract. these estimates should then be locked against change and I suppose should be check marked completed and submitted. All of these are the old estimates. The current estimate is the one in progress., The new one that is just starting. To answer your question each have to be saved as a separate entity (by estimate ID). the older estimates will need to be in a drop down list but current and new have a single choice each.
Tom

You could add a Status to the estimate