As a general rule, the more you deviate from what is written on the ticket, the harder it will become to get it accepted....

Normally, swapping airlines on the route the ticket is issued for is widely accepted (on condition the ZED class is the same or higher) and most airlines will also accept a ticket to another destination than the one the ticket was initially issued on (on condition the ZED fare is the same or higher), but to assume you can combine these 2 may be a bridge too far....

Personally, I wouldn't try it, especially not since you're not just willing to change airline AND destination, but also point of departure...

If you want to be sure, try to ask bmi at BGO well in advance as they are the ones that take the final decision.