Because you talked about having to change the back cover I had assumed that you had an older Beocentre 1.
There was four (4) versions of the Beocentre 1 as follows:-
- Beocentre 1 Type 850X which does not have AC3/DTS module and it is not available as an option
- Beocentre 1 Update 1 Type 851X which does not have AC3/DTS module and it is not available as an option
- Beocentre 1 Update 2 Type 852X can have a type 4063 AC3/DTS surround sound module as an option
- Beocentre 1 Update 3 Type 8531 to 8540 can have a type 4063 AC3/DTS surround sound module as an option
The Beovision 3 can have a Dolby Digital module as an option and is type 4003
So, I stand by my answer that you CANNOT use a 5:1 module from your Beovision 3 in your Beocentre 1 of any version.
You should check the 4 digit "Type" number of your Beocentre 1 against the list above.
If you have the later version of the Beocentre 1, then you CAN purchase it as an option.