Hi.
The newer 850ci has an M73 engine (5.4litre) which is an aluminum V12, the older one an M70 engine (5.0 litre), non aluminum.
How do you know if you have the M73 or M70 engine:
The 850ci M73 (1994>) only comes with a auto 5speed transmission, so if you don't have a 5spd auto, you're driving a 850i M70 (<1994).

Here are the 2 850(c)i's next to each other (the picture is the same):
http://www.bmwheaven.com/index.php?option=com_bkbsearch&func=bkbcompare&Itemid=9&brand1=BMW&enumber1=e31_8&type1=453&brand2=BMW&enumber2=e31_8&type2=454&submit=compare

Think you can change interior and exterior parts, block would be a lot harder.
Good news about your 850i M70 engine:
- it can be converted to 6.0L, the M73 cannot
- The M70 is more turbo friendly than the m73 because of compression ratios

Yes, the M70 was also built in 1994.
This is what I got off wikipedia:
"850Ci

There is some confusion over why and when the 850i became the 850Ci. The change happened around the introduction of the 850CSi and it is believed that BMW decided to include the "C" in the model name to create more of a distinction between the 8 Series and BMW's sedans. The confusion starts when BMW installed the new M73B54 engine in the car. This was not an immediate changeover, and indeed both the M70 and M73-engined cars rolled off the production lines side-by-side for about nine months in 1994, both named 850Ci.

As the capacity of the M73 increased to 5.4 liters and the compression ratio went up, the power output rose to 326 bhp (243 kW/331 PS)"