If my contract starts at 01.12.2004, I will be GC then at beginning of 2005 it will change automatically to AE? or I need to get into many difficult paper work?

One more thing, i would like the advice in this matter:

what is more easier to do and which is more advisable for my side and for my companies side: To do the contract at 01.10.2004 and change it afterwards to AE or to wait till 01.01.2005 and start it directly AE???

Well, if you start with a GC before 01.01.2005 AND the regulations stipulate that a GC is candidate for a WP then you won't need to take integration courses, and getting a Permanent residence permit will be easier.

If the regulations stipulate that the GC will not be able to stay, thn you may have to leave after 5 years.

If you start with an AE after the 01.01.2005 and you stay here for 5 years, you will get a permant residence Permit, but you will have to take integration courses and maybe a language test, except if your contract is for around 85K euros, in which case you can get an unlimited residence permit from the beginning.