Change in input format destroys php code

Not sure if this is the right issue forum.

I added a new field to the page content type. The default input format was PHP code so I could add an include statement to some pages.

Unfortunately while fooling around with settings I must have changed the input format to plain text or something. The result was that I lost the php statements in the pages. This cost me a couple of hour's work.

It would have been great if there had been a warning statement saying something like Changing Format Will Zap Your Code!!!

Thanks