One thing is for sure. If you are using other backend languages then english the update will not work because the 4.3 language resources are not ready!!
So if you use different backenlanguages you need to do the following.
Change backend default language to EN as per this KB article http://www.sitefinity.com/devnet/kb/sitefinity-4-x/telerik-web-ui-parseexception-expression-expected-exception-after-upgrade-to-sitefinity-4-3.aspx
Then you could use the 4.3 update. Once the 4.3 lanugage files are released you can import them and remove the code from the KB article.
Since all my sites have DE backend languages I was not able to test 4.3 so I am not able to help you on your question. Sorry.
It really would be nice if Telerik would be specific about this matter from what version you can update. Maybe its out there and I just missed it.