At the start of a project, the codes(and the revision) to be used are put into the contract documents. Some of these projects can take months to complete and others may take years to complete. Codes can be revised and edited anytime while these projects are ongoing, but the contract documents/project specifications will govern the code and it's revision to be used during the construction of the project.
So let's say the contract documents say to use AWS D1.1:2010, even though there is a newer revision of AWS D1.1 out in print(currently 2015), the 2010 revision of the code will be used for the duration of the project. Check out the Scope in Clause 1.
edit: I'm assuming that you are asking if welds that were inspected and accepted under the older revision need to be reinspected under the new revision....answer is no, see above explanation