I was using PCS7 7.1 SP1 and updated to SP3. Project was converted, everything is ok.
I was looking into the manual about APL. I needed picture: @PCS7elementsAPL.pdl, but I couldn't find it.This picture is includedin the project from SP2 and above, in SP1 and lower, this picturedoesn't exist and it doesn't seem to be updated automatically.

So I found in directory: Siemens\Wincc\Options\PDL\apl, the picture (size 3348kb) and copied it into my project.
Everything should be ok now. When starting a new project, the picture is already included..

Grtzz

In the "PCS 7 Libraries APL Style Guide"
http://support.automation.siemens.com/WW/view/en/43705259
it is described, that the "@PCS7ElementsAPL.pdl" is new since SP2.

If you update form PCS 7 V7.1 or PCS 7 V7.1 SP1 to SP3 this picture will be installed.

Did you run the Project Editor?

Regards