I have a rather large PHP script written in PHP 5.3, but I have now moved to a new server that has 5.4.

Is there away in which I can make the PHP 5.4 backward compatible.

Failing that is there an easy way in which I can convert the script I have, which is well over a thousand files into 5.4

I really don't want to start over.

As always, any help is appreciated.

I think those are just safety and security improvements. If your code is clean way and well coding habit guarded (i.e. if you have coded nice bridge if required file doesn't exists), there is no need to worry about.
Here you have changing log and list of what is done in 5.4.0 version.
Also, those are changes related to 5.3.10 subversio. Check what exact (sub)version was your 5.3 on the same link and compare changes to it.

Thught I had responded, but my Internet connection must have dropped (the joys of living out of a case)