It's fairly common for clients to make small adjustments to the source text after we have started translating a document. In many cases, these adjustments consist of spelling mistakes and inconsistencies that one of our translators flag. In such cases, we just need to check that the meaning of the text doesn't change and the translation doesn't need to be updated. In other cases, the client is waiting for the final confirmation of certain phrases. This is all fine and we are always happy to incorporate updates into the translation.
However, we are sometimes asked to work on a document that hasn't been finalized internally. We are reluctant to start on such translations because the source text can sometimes change dramatically. Amending the translation according to the new source text might end up costing more and taking longer than waiting for the final version of the text.
If you need an urgent translation, please have a look at When is my project considered and charged as 'urgent'?.