Changing a 530d back to legal tax????

Is there much involved I changing a logbook from 2 litre back to 3 litre, I have my eye on an e39 but want to have everything above board if I do buy it.

Any help/advice would be great.

Thanks
Johny.

You need to bring it to bmw, get them to inspect it and give u a letter stating the correct engine cc and engine number and chassis no, theyl prob charge ya an hours labour for it, then into tax office and theyl change it for u, big problem now is that most cars thats have had cc changed on logbook in last 3 years, when they are sent i to shannon for change of ownership they are now holding the book and will not process the change, until the ownerr provides proof from dealer of cc and engine no, i know 2 or 3 people this has happened to 2 bmws and a skyline, so they bought the cars and the book never arrived, alot of hassle then trying to get last owner to sort it for it fr ya.
My advice would be to get the current owner to change it back first, or get him go give you the book when buying it, as if that book goes to shannon with cc on it you wont get it

just to explain then, if he gives you the book(which he should considering hes going to find it hard to sell otherwise) go to bmw with a change of particulars form, get them to inspect it, stamp the change or particualrs form, AND give you a headed letter with engine no cc and chassis no on it, then go to tax office tell them youve just bought it, got it inspected, and you want to change ownership engine cc and tax it, theyl do it all there and then for you. but you must tax it, if you wish not to tax it theyl send it all to shannon and will take much longer !!