>>>Do you know utility, which would help me to do this?
>>>
>>>Thanks a lot in advance.
>>
>>What's wrong with this?
>>
>>Add class a of a.vcx to b.vcx
>>
>>You get a new copy of the class with everything.
>
>Because this way I may hit the same problem I'm currently having with this class. However, I'm going to try it tomorrow, may be it would be enough.

Just as you would do with editing and adding the PEMs one by one from the old class, you can do the same, just in reverse, with a copy where you would remove things one by one until you find out which one was the cause.